Three-dimensional thin film semiconductor substrate with through-holes and methods of manufacturing

ABSTRACT

A three-dimensional thin-film semiconductor substrate with selective through-holes is provided. The substrate having an inverted pyramidal structure comprising selectively formed through-holes positioned between the front and back lateral surface planes of the semiconductor substrate to form a partially transparent three-dimensional thin-film semiconductor substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/962,291 filed Aug. 8, 2013 which is a continuation of U.S. patentapplication Ser. No. 12/791,842 filed Jun. 1, 2010 issued as U.S. Pat.No. 3,551,866 which claims the benefit of both U.S. Prov. App.61/182,635 filed on May 29, 2003 and U.S. Prov. App. 61/228,068 filed onJul. 23, 2009, all of which are hereby incorporated by reference intheir entirety.

FIELD OF THIS INVENTION

This disclosure relates in general to the field of photovoltaics andsolar cells, and more particularly to semiconductor substrates andmethods for making semiconductor substrates with through-holes for usein manufacturing three-dimensional thin-film solar cells.

BACKGROUND OF THE INVENTION

Building-integrated Photovoltaics (BIPV) involves the process ofintegrating energy (electricity) producing photovoltaic technology intoresidential, commercial, and industrial building and construction designand materials. By using BIPV, the solar electricity producing componentsactually become an integral part of the building or constructionmaterials and design, often serving as the protective exteriorweathering skin, and/or interior building components. Semi-transparentor see-through solar PV modules comprise the most attractive segment ofBIPV applications. These modules can be used in many applicationsincluding window glazing in building windows. In some applications theyare also a part of shading devices such as car parking covers. Such BIPVsystems are also known as “shadow-voltaic” systems. See-through BIPVmodules can be also part of energy efficient glaring, where they areused instead of usual glass.

Currently, about 80% of the BIPV applications are served by crystallinesemiconductor cell technology, while inorganic thin-film (TF)technologies account for the remaining 20% of the total BIPV market.However, the TF technologies are projected to capture over 50% of theBIPV applications by 2015. The TF technologies include amorphoussemiconductor (a-Si), cadmium telluride (CdTe),copper-indium-galium-diselenide (CIGS), and organic PV. Among them, CdTeand CIGS promise higher efficiencies than a-Si. However, these TFtechnologies in practice offer efficiencies in the range of 5% to 12%,with the TF see through BIPV module efficiencies being essentiallylimited to the single-digit efficiency range of 4% to 8%. Both organicPV (OPV) and dye-sensitized solar cells (DSSC) are considered to be thethird generation BIPV technologies (both currently providing moduleefficiencies on the order of 6%). All the TF and DSSC BIPV technologiescurrently offer much lower efficiencies than crystalline semiconductorBIPV. However, the TF technologies provide better aesthetics thancrystalline semiconductor, particularly for see-through BIPV moduleapplications. In a typical see-through crystalline semiconductor BIPVmodule for solar glass applications, the crystalline semiconductor cellsare spaced apart to allow for visible light transmission in between thetiled cells. While this see-through crystalline semiconductor PV modulescan provide relatively high effective efficiencies (e.g., typically inthe range of 10% to 12%), they do not offer very attractive aesthetics,both due to the tiled design and also due to the standard busbar emitterinterconnects in the cells (thus, showing visible metallization fingersand busbars).

Dye-sensitised solar cells (DSSC) operate based on the interactionbetween light and a dye coated onto small grains of titanium dioxide.The grains are placed in a liquid that acts as an electrolyte,collecting the electrons released by the dye as it absorbs light, thus,generating current. The whole mixture is sandwiched between atransparent glass sheet electrode doped with tin oxide to make itelectrically conducting, and a rear panel. The efficiency of DSSCdesigned for outdoor conditions is currently about 6%. This is far belowthe efficiency of standard crystalline semiconductor BIPV modules.

OPV and DSSC BIPV modules cannot easily compete with the conventionalcrystalline semiconductor or TF BIPV solar panels due to theirrelatively low conversion efficiencies and shorter operationallifetimes. Crystalline semiconductor solar cells and modules have provenlong lifetimes in excess of 25-30 years in the field and no TF or DSSCtechnology can offer or match such track record. While the conventionalcrystalline semiconductor wafer BIPV is only suitable for rigid BIPVapplications, the TF and DSSC BIPV modules can be used for both rigidand flexible substrate applications.

This invention provides a 3D crystalline (including mono-crystalline)thin-film semiconductor substrate for making disruptive,high-efficiency, and low cost see-through solar cells. The semiconductorfilm thickness may be in the range of a few microns to tens of microns(up to ˜100 μm).

For example, U.S. Pat. Pub. No. 2008/0264477, U.S. Pat. Pub. No.2008/0289684, U.S. Pat. Pub. No. 2008/0295887 and U.S. Pat. Pub. No.2009/0107545 by common inventor Mehrdad M. Moslehi disclose methods formanufacturing a 3-Dimensional Thin-Film Soar Cell (3-D TFSC). Themethods comprise forming a 3-Dimensional Thin-Film Substrate (3-D TFSS)using a semiconductor template. The template structures may comprise anycombination or variation of three-dimensional surface features such as aplurality of posts and a plurality of trenches between said a pluralityof posts or a plurality of inverted three-dimensional pyramid surfacecavities. The 3-D TFSS is forced by forming a sacrificial layer on thetemplate, subsequently depositing a semiconductor layer, selectiveetching the sacrificial layer and releasing the semiconductor layer fromthe template. More specifically, the semiconductor layer is aself-supporting, free-standing three-dimensional (3D) epitaxialsemiconductor thin film deposited on and released from a low-costreusable crystalline semiconductor substrate template. The reusablesemiconductor template may be reused to form the 3D film numerous timesbefore being reconditioned or recycled. Select portions of the released3-D TFSS are then doped with a first dopant, and other select portionsare than doped with a second dopant. After surface passivationprocesses, emitter and base metallization regions are formed to completethe solar cell structure.

Known 3-D TFSS fabrication methods provides fabrication processimprovements and manufacturing cost reduction by using inverted andstaggered pyramid structures on the re-usable semiconductor templates.More specifically, the inverted pyramid structures disclosed in the saidpatent application are made by KOH-like semiconductor etching. The knowncrystallographic semiconductor planes on the template surface from KOHetching enable convenient epitaxial growth control as well as largenumber of template re-use cycles in order to amortize the template costover numerous released cell substrates.

SUMMARY OF THE INVENTION

In accordance with the present disclosure, methods for manufacturingthree-dimensional thin-film solar cells (3-D TFSCs) are provided. The3-D TFSCs of the disclosed subject matter substantially eliminate orreduce disadvantages and problems associated with previously developedsemiconductor wafer-based solar cells as well as TFSCs, both in terms ofconversion efficiency as well as manufacturing costs.

According to one aspect of the disclosed subject matter structuraldesigns and methods of manufacturing 3-D see-through TFSS and TFSCs aswell as aesthetically appealing, three-dimensional (3-D)Partially-Transparent (See-through) TFSCs with relatively highconversion efficiencies and substantially uniform lighttransmissivities. The disclosed subject matter is particularlyapplicable to Building-Integrated PhotoVoltaics (BIPV) applicationsincluding power-generating solar glass as well as other solarphotovoltaic power modules.

According to one aspect or the disclosed subject matter, there isprovided 3-D See-through Thin-Film-Semiconductor-Substrate (TFSS),comprising continuous, staggered, inverted pyramidal cavities.Furthermore, the pyramid structures may comprise of square and/orrectangular pyramids with two or more different sizes that areinterweaved for providing an overall better mechanical rigidity than a3-D TFSS with non-staggered, regular array pyramid structures. Thepyramid layout designs of the disclosed subject matter staggered andnon-staggered inverted pyramidal surface features.

According to another aspect of the disclosed subject matter, there isprovided partially transparent 3-D TFSS or crystalline film solarsemiconductor substrate, comprising staggered, inverted and hollowpyramid structures with uniformly distributed see-through apertures orholes etched through the pyramid structures.

Furthermore, the pyramid structures comprise of square and/orrectangular pyramids with two or more different sizes that areinterweaved for providing an overall better mechanical rigidity than a3-D TFSS with non-staggered, regular array pyramid structures. Thepyramid layout designs of the disclosed subject matter include thestaggered and non-staggered pyramid structures. The see-throughapertures or holes are etched (or chemically/mechanically polished) fromone or multiple sizes of the pyramids. The partially transparent 3-DTFSC are provided for BIPV applications such as electricity generatingsolar glass. The 3-D TFSC of this type is provided, for conventionalmodule (e.g., rooftop) and solar field (solar PV power farm)photovoltaic applications.

According to another aspect of the disclosed subject matter, there areprovided fabrication methods forming see-through 3-D TFSSs and 3-DTFSCs. Such as using abrasive tape and diaphragm lapping forself-aligned thin layer removal on a pre-structured wafer surface andusing abrasive tape lapping for wafer edge thin layer removal that isunique for the fabrication process of the present invention.

According to another aspect of the disclosed subject matter, there areprovided fabrication methods in making the see-through holes on the 3-DTFSS in self-aligned coating and etching processes. In addition to using3-D TFSSs with see-through hole for fabricating partially transparent3-D TFSCs, the see-through holes, with proper dimension and locationdesigns, may also be used to make 3-D TFSCs with solar cell emitter andbase contacts on the same side of the substrate. In this case,through-TFSS emitter metal plugs may be made using the see-through holesto make emitter wrap-through or back-contact cells.

The 3-D TFSS is formed by forming a bi-layer, triple-layer, orgraded-porosity porous semiconductor layer (a low porosity layer on topof a high porosity layer) on a pre-structured semiconductor template,subsequently depositing a semiconductor layer by epitaxial semiconductorgrowth and then releasing the semiconductor layer from the template. Thesemiconductor template, which comprises a plurality of pre-structuredinverted pyramid structures, may be reused to form the 3-D TFSS numeroustimes before being recycled. Select portions of the 3-D TFSS is dopedwith n-type and p-type dopant during the epitaxial growth and form theemitter and base of the semiconductor solar cell in the in-situepitaxial growth process. Alternatively, the emitter doping process maybe performed after the epitaxial semiconductor growth or even after theTFSS releasing. Next, front surface passivation andanti-reflection-coating (ARC) as well as backside passivation layer areformed followed by opening of the dielectric layer to form the emitterand base metal contact regions by subsequent metallization processes(such as with selective plating). In the case of partially transparentTFSCs, the see-through holes are preferably formed prior to the surfacepassivation and metallization processes.

The 3-D TFSCs may toe made to be relatively rigid, semi-rigid, orflexible depending on the structural design parameters of the cellsubstrate. This structural design and fabrication process results insubstantially reduced semiconductor consumption and PV manufacturingcost. In addition, compared to other flat, thin-film (TF) crystal lineSi approaches, 3D-TFSC in the present invention cell offers thefollowing hey advantages: (i) Surface texturing of flax, thin film, Sito reduce reflectance losses requires a minimum film thickness ofpreferably tens of microns (e.g., >30 μm) to avoid texturingetch-induced punch-through pinholes. 3D-TFSC of the disclosed subjectmatter does not suffer from this constraint. (ii) Thin flat Si filmshave much reduced mean optical path length which reduces IR absorption,and results in reduced cell quantum efficiency. 3D-TFSC of the disclosedsubject matter traps light extremely efficiently by virtue of its 3Dnature. (iii) Flat TF Si has poor mechanical strength for ceil andmodule processing needs. 3-D TFSC disclosed has proven to bemechanically robust because of its unique 3D structure, providingenhanced mechanical strength and handlebility. (iv) The staggered,inverted and hollow pyramid structure allows see-through holes to bemade in a convenient self-aligned patterning and etching method.

These and other advantages of the disclosed subject matter, as well asadditional novel features, will be apparent from the descriptionprovided herein. The intent of this summary is not to be a comprehensivedescription of the claimed subject matter, but rather to provide a shortoverview of some of the subject matter's functionality. Other systems,methods, features and advantages here proved will become apparent to onewith skill in the art upon examination of the following Figures anddetailed description. It is intended that all such additional systems,methods, features and advantages be included within this description, bewithin the scope of the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the disclosed subject matter andadvantages thereof, reference is now made to the following descriptiontaken in conjunction with the accompanying drawings in which likereference numbers indicate like features and wherein:

FIGS. 1A through 3B show examples of templates and 3-D TFSS/TFSCswithout see-through holes;

FIG. 4A and 4B are SEM images of the front side (FIG. 4A) and back side(FIG. 4B) a 3-D TFSS with etched see-through holes;

FIG. 5A illustrates a top view of an embodiment of a 3-D TFSS withetched see-through holes;

FIGS. 5B and 5C are cross-sectional illustrations of the 3-D TFSS and3-D TFSC;

FIG. 6A illustrates a top view of an embodiment of a 3-D TFSS withetched through holes;

FIGS. 6B and 6C are cross-sectional illustrations of the 3-D TFSS and3-D TFSC;

FIG. 7A illustrates a top view of an embodiment of a 3-D TFSS withetched through holes;

FIGS. 7B and 7C are cross-sectional illustrations of the 3-D TFSS and3-D TFSC;

FIGS. 8A and 8B show alternative 3-D TFSS inverted pyramid cavitypatterns;

FIGS. 9 thru 13 are process flows outlining several embodiments forfabricating a 3-D TFSS with see-through holes;

FIG. 14 outlines another process flow for fabrication of a 3-D TFSC withor without see-through holes;

FIG. 15A through 15D Illustrate schematic cross-sectional drawings of atemplate fabrication process;

FIGS. 16A through 16C are cross-sectional drawings illustrating a 3-DTFSS fabrication process with in-situ emitter, base, and BSF doping;

FIGS. 17A through 17C are cross-sectional drawings illustrating a 3-DTFSS fabrication process in which neither the emitter nor BSF isparticularly doped during epitaxial growth;

FIGS. 18A through 18C are cross-sectional drawings illustrating a 3-DTFSS fabrication process in which neither the emitter nor back surfacefield (BSF) is particularly doped during epitaxial growth;

FIGS. 19A through 19C illustrates a selective emitter and BSF dopingprocess performed on a released 3-D TFSS by liquid transfer coating;

FIGS. 20A through 20D the 3-D TFSS/TFSC see-through fabrication processin which the small pyramids will be selectively patterned and etchedfrom their backsides;

FIGS. 21A through 21D the 3-D TFSS/TFSC see-through fabrication processin which the small pyramids will be selectively patterned and etchedfrom their backsides;

FIGS. 22A through 22D the 3-D TFSS/TFSC see-through fabrication processin which the large pyramids will be selectively patterned and etchedfrom their backsides;

FIGS. 23A through 23D the 3-D TFSS/TFSC see-through fabrication processin which the flat surface areas will be selectively patterned and etchedfrom their frontside;

FIGS. 24A through 24D are cross-sectional drawings depicting a processflow for the fabrication a 3-D TFSC with or without see-through holes;

FIGS. 25A through 25D are cross-sectional drawings depicting a processflow for the fabrication a 3-D TFSC with or without see-through holes;

FIGS. 26A and 26B are a cross-sectional depiction illustrating liquidtransfer coating with a roller coater and the resulting 3-D TFSS;

FIGS. 27A and 27B are a cross-sectional depletion illustrating liquidtransfer coating according to a stamping/dipping process;

FIGS. 28A and 28B are a cross-sectional depiction illustrating aselective and self-aligned mechanical polishing/lapping process;

FIGS. 29A and 29B are a cross-sectional depiction illustrating aselective and self-aligned mechanical polishing/lapping process;

FIGS. 30A and 30B are a cross-sectional depiction illustrating aselective and self-aligned mechanical polishing/lapping process;

FIGS. 31A and 31C are a cross-sectional depiction illustrating atemplate/wafer edge lapping process;

FIG. 32 is a process flow outlining the major steps of an overallre-usable template, 3-D TFSS and TFSC fabrication process with selectivebase metal, epitaxial emitter and plated metallization; and

FIG. 33 is a process flow outlining the major steps of an overallre-usable template, see-through 3-D TFSS and TFSC fabrication processwith selective base metal, epitaxial emitter and plated metallization.

DETAILED DESCRIPTION

The following description is not to be taken in a limiting sense, but ismade tor the purpose of describing the general principles of the presentdisclosure. The scope of the present disclosure should be determinedwith reference to the claims. Exemplary embodiments of the presentdisclosure are illustrated in the drawings, like numbers being used torefer to like and corresponding parts of the various drawings.

Although described with reference to specific embodiments, one skilledin the art could apply the principles discussed herein to other areasand/or embodiments. A preferred semiconductor material for the 3-D TFSSis crystalline silicon (α-Si) although other semiconductor materials mayalso be used. One embodiment uses monocrystalline silicon as the thinfilm semiconductor material. Other embodiments use multicrystallinesilicon, polycrystalline silicon, microcrystalline silicon, amorphoussilicon, porous silicon, and/or a combination thereof. The designs hereare also applicable to other semiconductor materials including but notlimited to germanium, silicon germanium, silicon carbide, a crystallinecompound semiconductor, or a combination thereof. Additional,applications include copper indium gallium selenide (CIGS) and cadmiumtelluride semiconductor thin films.

Further, although the disclosure has typically described P and Bimplantation for n and p doping, respectively, other elements such as Asand Sb may be used for n doping, and Al, Ga, In, may be used for pdoping.

In the present application, the term 3-D TFSS refers to athree-dimensional thin-film semiconductor substrate and the term 3-DTFSC refers to a three-dimensional thin-film solar cell. Further theterm selective refers to the controlled etching or coating of selected,surface regions on a template, 3-D TFSS, or 3-D TFSC. Further, the termflat surface areas may refer to the top ridges of the inverted pyramidalcavities or larger flat top surface areas on the top surface of the 3-DTFSS and 3-D TFSC.

FIGS. 1A through 3B show examples of templates and 3-D TFSS/TFSCswithout see-through holes.

FIG. 1A illustrates a top Scanning Election Microscope (SEM) partialview of an embodiment of an etched re-usable semiconductor template withstaggered inverted pyramidal cavities as shown in U.S. patentapplication Ser. No. 12/618,663 having common inventors David Xuan-QiWang and Mehrdad M. Moslehi. Structured semiconductor template 10consists of anisotropically etched small pyramid cavities 12 and largepyramid cavities 14. The top opening size 16 of the large cavities is inthe range of 10 um to 1 mm and the top opening size 18 of the smallcavities is a partial of 16. As shown the opening size of the largecavities, 16, is about 300 um and opening size of the small cavities,18, is about 150 um. In this case, the depth of the small cavities isabout 110 um and the depth of the large cavities is about 200 μm. Thetop surface ridges between the inverted pyramid cavities, shown asreference numeral 20, are aligned along the (100) semiconductorcrystallographic planes. The sidewalls of the cavities are aligned along(111) semiconductor crystallographic planes. Small pyramid cavity 12 hasa pointed cavity apex/bottom which occurs at the intersection of fourfill (111) sidewalls. Large pyramid cavity bottom 14 has a flat cavitybottom/apex the size of which may be in the range of 0 to 100 um and isaligned along the (100) crystallographic plane. The ridge defining theopening between inverted pyramidal cavities, 20, is preferably narrowand less than 10 um wide.

The semiconductor template defines the basic structures of the 3-D TFSCsand it is used in multiple re-use cycles to make 3-D TFSSs. In otherwords, one template could be used to produce many substrates (3-D TFSSs)for reducing the mono-crystalline semiconductor consumption and cost.The semiconductor template is anisotropically etched from a p-type 200mm semiconductor wafer from a batch etching process and its frontsideconsists of inverted and staggered large and small pyramid cavities asshown in FIG. 1A. The width of the pyramid cavity is in the range of 50um to 100 um. The size of the small cavity is a fraction of the largecavity.

FIG. 1B is a graphic depiction of the template in FIG. 1A showing theC-C cross sectional line used in FIG. 1C. FIG. 1C is a graphic depictionof a cross section of the template in FIG. 1B along the C-C line.Inverted pyramidal template 28 consists of large inverted pyramidalcavities 30 and small inverted pyramidal cavities 32 which are definedby walls aligned along a (111) plane of inverted pyramidal template 28.Top surface ridge 34 is aligned along a (100) plane of invertedpyramidal template 28. The inverted pyramid-shape cavities may bechemically etched by anisotropic semiconductor etchant and the anglebetween a sidewall and a top lateral plane (such as top surface ridge34), is about 54.7°—which is the defined angle between two (111) and(100) semiconductor crystallographic planes. Shown, small invertedpyramidal cavities 32 have an apex defined by walls aligned along the(111) crystallographic planes giving the apex, or tip, of the invertedpyramid an angle of 70.6°. Large inverted pyramidal cavities 30 have aflat apex, or flat tip, aligned along the (100) crystallographic plane.However, the template may employ various apex styles dependent on shapeof the 3-D TFSS desired. It is to be noted the bottom of the largepyramid cavity may be etched to a sharp profile where the four sidewalls((111) semiconductor planes) meet or leave a small flat bottom surface((100) semiconductor plane).

FIGS. 2A and 2B are SEM images of a non-see-through 3-D TFSS and 3-DTFSC, such as those shown in U.S. patent application Ser. No. 12/618,663having common inventors David Xuan-Qi Wang and Mehrdad M. Moslehi. FIGS.2A and 2B show the SEM front side view (FIG. 2A) and back side view(FIG. 2B) of the 3-D TFSS prior to passivation layer coating andmetallization or process steps. The 3-D TFSS is preferably made ofmono-crystalline epitaxial grown semiconductor and either the front sideor the back side of the 3D-TFSS may face the sun light during use. Itcomprises of staggered/interweaved, inverted and hollow pyramids thathave two or more different sizes. Since the 3-D TFSS is made andreleased from a semiconductor template, the backside of the 3-D TFSS(shown in FIG. 2B) resembles the inverted surface profile of thesemiconductor template top surface shown in FIG. 1A. The thickness ofthe deposited epitaxial layer is in the range of a 5 to 50 microns.Because of its 3-D pyramid structural design, the thickness of the 3-DTFSS is in the range of 100 um to 500 um. The area of each 3-D TFSC isin the range of 100 cm² to 250 cm². The shapes of the 3-D TFSCs mayinclude squares, squares with truncated or rounded corners, andhexagons.

FIG. 3A is cross-sectional depiction of the 3-D TFSS shown in FIG. 1Balong the C-C lines, tightly doped p-type 3-D TFSS 50 has n-type dopedemitter region 52 and p-type doped base region (Back Surface Field—BSF)54. The n-type and p-type region is preferred to be doped in-situ to theepitaxial semiconductor growth. Alternatively, these regions may bedoped by vapor. Liquid or solid phase dopant diffusion or directly byion implantation.

FIG. 3B is cross-sectional depiction of a 3-D TFSC made from the 3-DTFSS of FIG. 3A. 3-D TFSC 60 has n-type doped emitter region 62 andp-type doped base region (Back Surface Field—BSF) 64. 3-D TFSC 60 alsocomprises front side surface passivation layer 66 and antireflectioncoating (ARC) dielectric layer 70, back side surface passivation layer68, emitter metal contact 12 and base metal contact 76, and the backsidemetal reflector layer 74. The front and back side surface passivationlayers and the front ARC layer (which may be the same layer used forfrontside surface passivation) include, but are not limited to, a thinlayer of or a combination of layers of thermal oxide, semiconductornitride layer and/or aluminum oxide. The dielectric layers on front andback surface may be selectively opened for making the emitter and basemetal contacts. In order to further reduce electrical, seriesresistance, the contact regions of semiconductor may bedifferentially/selectively doped with n+ for emitter and p+ for base.The choice of emitter metals includes but is not limited to fire-throughsilver (Ag) or electroplated/electroless-plated nickel (Ni) and silver(Ag). Alternatively, without opening the dielectric layers, the metalcontacts may be formed by firing through the oxide and nitride layers.The backside reflector layer can be made of a thin layer ofPVD/sputtered aluminum (Al), silver (Ag), or a dielectric(oxide/nitride) layer with a particular thickness to reflect the longwavelength light that penetrate to the back surfaces.

FIG. 4A and 4B are SEM images of the front side (FIG. 4A) and back side(FIG. 4B) a 3-D TFSS with etched see-through holes in accordance withthe disclosed subject matter. Shown, the 3-D TFSS is made ofmono-crystalline, epitaxial grown semiconductor and the front side ofthe 3D-TFSS (shown in FIG. 4A) faces the sunlight during use. Before thesee-through holes, such as see-through hole 80, are etched, the 3-D TFSSin FIG. 4A and 4B has a substantially similar 3-D surface structure asthe non-see-through 3-D TFSS described and illustrated in FIGS. 2A and2B—in which the 3-D TFSS is comprised of staggered/interweaved, invertedpyramidal cavities of two or more different sizes.

FIG. 5A illustrates a top view of an embodiment or a 3-D TFSS withetched see-through holes in accordance with the disclosed subjectmatter. The 3-D TFSS is made of mono-crystalline epitaxial growthsemiconductor and the front side of the 3-D TFSS faces the sunlightduring use. FIG. 5A illustrates 3-D TFSS 90 with a type of an invertedand staggered pyramidal design consisting of staggered large pyramidcavities 94 and small pyramid cavities 92. The width of the pyramidcavity is in the range of 50 um to 500 um. The size of the small cavityis fraction of the large cavity. Prior to passivation layer coating andmetallization process steps, see-though holes 96 are etched into thesmall pyramids, preferably from the backside of 3-D TFSS 90 in acontrolled semiconductor etching process. The etched openings are small,in the range of 50 um to 300 um, and they are uniform and evenlydistributed among the large pyramids. Therefore, viewing by human eyes,either in close proximity to in a distance, the see-through effects ofthe resulting 3-D TFSC are aesthetically appealing. As in thenon-see-through 3-D TFSS described herein, the thickness of thedeposited, epitaxial layer is preferably in the range of 5 to 60microns. Because of its 3-D pyramid structural design, the peak-to-peakthickness of the 3-D TFSS is in the range of 100 um to 500 um. The areaof each 3-D TFSC is typically in the range of 100 cm² to over 300 cm².And the shapes of the 3-D TFSCs may include squares, squares withtruncated or rounded corners, and hexagons.

FIGS. 5B and 5C are cross-sectional illustrations of the 3-D TFSS and3-D TFSC, respectively, along the C-C cross-sectional line in FIG. 5A.FIG. 5B is cross-sectional depiction of the 3-D TFSS shown in FIG. 5Aalong the C-C lines. Lightly doped p-type 3-D TFSS 102 has n-type dopedemitter region 100 and p-type doped base region (Back Surface Field—BSF)104. Importantly 3-D TFSS 102 comprises see-through holes 106 that areetched from the back side of 3-D TFSS 102. The n-type and p-type regionis preferred to be doped in-situ to the epitaxial semiconductor growth.Alternatively, these regions may be doped by vapor, liquid, or solidphase dopant diffusion or directly by ion implantation. The n-type andp-type regions are preferably doped in situ during the epitaxialsemiconductor growth.

Alternatively, the n-type and p-type regions may be doped by vapor,liquid or solid phase dopant diffusion or directly by ion implantation.As illustrated in FIG. 5B, the lateral plane of the etch hole openingslies in between the front and back TFSS lateral planes. In other words,the etch hole opening is located between the pyramid top ridges andbottoms tips in the vertical direction. Therefore, the existence of thehole-openings does not interfere with the emitter and back metalcontacts. As a result, the fabrication process of the see-through 3-DTFSC is highly harmonized with the non-see-through 3-D TFSC. As anotheradvantage, the opening size or the ratio of opening area versusnon-opening area may be tuned to a specific value per design andcontrolled by convenient etching time control during the hole-openingetching process.

FIG. 5C is cross-sectional depiction of a 3-D TFSC made from the 3-DTFSS of FIG. 5B. 3-D TFSC 112 has n-type doped emitter region 110 andp-type doped base region (Back Surface Field—BSF) 114. 3-D TFSC 112 alsocomprises front side surface passivation layer 118 and antireflectioncoating (ARC) dielectric layer 124, back side surface passivation layer120, emitter metal contact 126 and base metal contact 128, and thebackside metal reflector layer 122. Importantly, 3-D TFSC 112 comprisessee-through hole 116 in accordance with the disclosed subject matter.The front and back side surface passivation layers and the front ARClayer include, but are not limited to, a thin layer of or a combinationof layers of thermal oxide, semiconductor nitride layer and/or aluminumoxide. As shown in FIG. 5C, the sidewalls of the etched see throughopening 116 are also covered with the passivation dielectric layer;therefore shorting between emitter and base metal contacts through theopenings is avoided. The dielectric layers on front and back surface maybe selectively opened for making the emitter and base metal contacts. Inorder to avoid large series resistance, the regions of semiconductorclose to the point contacts are differentially/selectively doped with n+for emitter, and p+ for base. The choice of emitter metals includesplated nickel (Ni) and silver (Ag). Alternatively, without opening thedielectric layers, the metal contacts may be formed by applying Ag andAl pastes to the front side (emitter) and backside (base) and formingmetallization via firing through the oxide and nitride layers. Thebackside reflector layer may be made of a thin layer of PVD/sputteredaluminum (Al), silver (Ag), or a suitable dielectric (oxide/nitride)layer with a particular thickness (with or without a metallic mirroroverlayer) to reflect the long wavelength (red and infrared) light thatpenetrate to the back surfaces.

FIG. 6A illustrates a top view of an embodiment of a 3-D TFSS withetched through holes in accordance with the disclosed subject matter.The 3-D TFSS is made of mono-crystalline epitaxial grown semiconductorand either the front side or the back side of the 3D-TFSS may face thesun light during use. FIG. 6A illustrates 3-D TFSS 140 with a type of aninverted and staggered pyramidal design consisting of staggered largepyramid cavities 144 with through holes 146 and small pyramid cavities142.

The see-through 3-D TFSS in FIG. 6A is similar to the 3-D TFSS describedand illustrated in FIG. 5A which is comprises of staggered/interweaved,inverted pyramidal cavities that have two or more different sizes;however, instead of having the see-through holes etched in the smallpyramid cavities, as in FIG. 5, the see-through holes are etched in thelarge pyramid cavities in an alternative see-through hole etchingprocess. As illustrated in FIG. 5A and 5B, prior to the passivationlayer coating and metallization process steps, see-though holes areselectively etched into the large pyramids preferably from the backsideof the 3-D TFSS in a controlled semiconductor etching process. Theetched openings are small, in the range of 50 um to 300 um, and they areuniform and evenly distributed among the large pyramids. Therefore,viewing by human eyes, either in close proximity or in a distance, thesee-through effects of the resulting 3-D TFSC are uniform andaesthetically appealing. As in the non-see-through case described, thethickness of the deposited epitaxial layer is in the range of 5 to 50microns. Because of its 3-D pyramid, structural design, the thickness ofthe 3-D TFSS is typically in the range of 100 um to 500 um. The area ofeach 3-D TFSC is in the range of 100 cm² to >300 cm². And the shapes ofthe 3-D TFSCs may include squares, squares with truncated or roundedcorners, and hexagons.

FIGS. 6B and 6C are cress-sectional illustrations of the 3-D TFSS and3-D TFSC, respectively, along the C-C cross-sectional line in FIG. 6A.FIG. 6B is cross-sectional depiction of the 3-D TFSS shown in FIG. 6A along the C-C lines. Lightly doped p-type 3-D TFSS 152 has n-type dopedemitter region 154 and p-type doped base region (Back Surface Field—BSF)156. Importantly 3-D TFSS 152 comprises see-through holes 158 that areetched from the back side of 3-D TFSS 152.

The n-type and p-type regions are preferably doped in situ during theepitaxial semiconductor growth. Alternatively, they could be doped byvapor, liquid or solid phase dopant diffusion or directly by ionimplantation. As illustrated in FIG. 6B, the lateral plane of the etchhole openings lies in between the front and back TFSS lateral planes. Inother words, etch hole opening 158 is located between the pyramid topridges and bottom tips in the vertical direction. Therefore, theexistence of the hole-openings does not interfere with the emitter andback metal contacts. As a result, the fabrication process of thesee-through 3-D TFSC is highly harmonized with the non-see-through 3-DTFSC. In another advantage, the opening size or the ratio of openingarea versus non-opening area may be purposely tuned to a specific valueper design and it is realized by convenient etching time control duringthe hole-opening etching process.

FIG. 6C is cross-sectional depiction of a 3-D TFSC made from the 3-DTFSS of FIG. 6B. 3-D TFSC 160 has n-type doped emitter region 162 andp-type doped base region (Back Surface Field—BSF) 164. 3-D TFSC 160 alsocomprises front side surface passivation layer 168 and antireflectioncoating (ARC) dielectric layer 174, back side surface passivation layer170, emitter metal contact 178 and base metal contact 176, and thebackside metal reflector layer 172. Importantly. 3-D TFSC 160 comprisessee-through hole 166 in accordance with the disclosed subject matter.

The front and back side surface passivation layers and the front ARClayer may include but are not limited to a thin layer of or acombination of layers of thermal oxide, semiconductor nitride layerand/or aluminum oxide. As shown in FIG. 6C, the sidewalls of the etchedopen are also covered with the passivation dielectric layer; thereforeshorting between tine emitter and base metal contacts is avoided. Thedielectric layers on front and back surface may be selectively openedfor making the metal contact. In order to achieve sufficiently lowelectrical series resistance, the contact regions ox semiconductor maybe differentially/selectively doped with n+ for emitter and p+ for base.The choice of emitter metals includes but not limited to plated nickel(Ni) and silver (Ag). Alternatively, without opening the dielectriclayers, the metal contacts may be formed by applying Ag and Al etchpastes on frontside and backside and then firing through the oxide andnitride layers to form the metal contacts. The backside reflector layermay be made of a thin layer of PVD/sputtered aluminum (Al), silver (Ag),or a dielectric (oxide/nitride) layer (with or without a metallic mirroroverlayer) with a particular thickness to reflect the long (red andinfrared) wavelength light that penetrates to the back surfaces.

FIG. 7A illustrates a top view of an embodiment of a 3-D TFSS withetched through holes in accordance with the disclosed subject matter.The 3-D TFSS is made of mono-crystalline epitaxial grown semiconductorand the front side or the back side of the 3D-TFSS may face the sunlight during use. FIG. 7A illustrates 3-D TFSS 180 with a type of aninverted and staggered pyramidal design consisting of staggered pyramidcavities 184 and flat surface areas 182 with through holes 186.See-through 3-D TFSS 180 comprises staggered/interleaved, invertedpyramidal cavities 184 that have flat surface areas 182 regularlydistributed among them. However, instead of having the see-through holesetched in the small pyramids as in FIG. 5A, see-through holes 186 areformed in the flat surface areas. As illustrated in FIG. 7A, prior topassivation layer coating and metallization process steps, see-thoughholes are formed in the flat surface areas from either the front or theback side of the 3-D TFSS. The etched openings are small, in the rangeof 50 um to 500 um, and they are preferably uniform and evenlydistributed, among the large pyramids. Therefore, viewing by human eyes,either in close proximity or in a distance, the see-through effects ofthe 3-D TFSC is aesthetically uniform and appealing. As in thenon-see-through case described, the thickness of the deposited epitaxiallayer is in the range of 5 to 50 microns. Because of its 3-D pyramidstructural design, the thickness of the 3-D TFSS is in the range of 100um to 500 um. The area of each 3-D TFSC is in the range of 100 cm² toover 300 cm². And the shapes of the 3-D TFSCs may include squares,squares with truncated or rounded corners, and hexagons.

FIGS. 7B and 7C are cross-sectional illustrations of the 3-D TFSS and3-D TFSC, respectively, along the C-C cross-sectional line is FIG. 7A.FIG. 7B is cross-sectional depiction of the 3-D TFSS shown in FIG. 7Aalong the C-C line. Lightly doped p-type 3-D TFSS 190 has n-type dopedemitter region 192 and p-type doped base region (Back Surface Field—BSF)194. Importantly 3-D TFSS 190 comprises see-through holes 196 that areetched from either the front or back side of 3-D TFSS 190. The n-typeand p-type regions are preferably doped in situ daring the epitaxialsemiconductor growth. Alternatively, they could foe doped by vapor,liquid or solid phase dopant, diffusion or directly by ion implantation.The opening size or the ratio of opening area versus non-opening areacan be tuned, when the opening pattern is made.

FIG. 7C is cross-sectional depiction of a 3-D TFSC made from the 3-DTFSS of FIG. 7B. 3-D TFSC 200 has n-type doped emitter region 202 andp-type doped base region (Back Surface Field—BSF) 204, 3-D TFSC 200 alsocomprises front side surface passivation layer 208 and antireflectioncoating (ARC) dielectric layer 212, back side surface passivation layer210, emitter metal contact 216 and base metal contact 218, and thebackside metal reflector layer 214. Importantly, 3-D TFSC 200 comprisessee-through hole 206 in accordance with the disclosed subject matter.

The front and back side surface passivation layers and the front ARClayer include but not limited to a thin layer of or a combination oflayers of thermal oxide, semiconductor nitride layer and/or aluminumoxide. As shown in FIG. 7C, the sidewalls of the etched openings orapertures are also covered with the passivation dielectric layer;therefore electrical shorting between the emitter and base metalcontacts is avoided. The dielectric layers on the front and back surfacemay be selectively opened for making the metal contact. In order toensure sufficiently low electrical series resistance, the contactregions ate differentially/selectively doped with n+ for emitter and p+for base. The choice of emitter metals includes but is not limited toplated nickel (Ni) and silver (Ag). Alternatively, without opening thedielectric layers, the metal contacts may be formed by applying silverand aluminum pastes and firing through the oxide and nitride layers. Thebackside reflector layer may be made of a thin layer of PVD/sputteredaluminum (Al), silver (Ag), or a dielectric (oxide/nitride) layer with aparticular thickness (with or without a metallic reflector overlayer) toreflect the long wavelength (red and infrared) right that penetrate tothe back surfaces.

In addition to the see-through structures illustrated in FIGS. 5A, 6A,and 7A, see-through holes may be made larger than the pyramid cavitysizes and/or coarsely distributed. Furthermore, the see-through openingsdo not have to be aligned to the pyramid patterns. In these irregularsee-through pattern layouts, even though the conversion efficiency maybe lower than the layouts described in FIGS. 6A, 6A, and 7A, theresulting 3-D TFSC may still be useful and provide higher efficiencythan other partially transparent solar cells currently known. As anexample, see-through holes may be conveniently drilled by laser ablationwith or without alignment to the pyramid cavity patterns. The laserdrilling may be done before or after the surface passivation andmetallization process steps. Methods of forming the see-through holesand layouts of the see-through holes of irregular types are also withinthe scope of the disclosed subject matter. Further, the disclosedsubject matter may be used for both mono-facial and bifacial see-throughsolar cells.

FIGS. 8A and 8B show alternative 3-D TFSS inverted pyramid cavitypatterns. 3-D TFSS 230 in FIG. 8A comprises small square cavity 234 andlarge rectangular cavity 232. And 3-D TFSS 240 in FIG. 8B comprises apattern of large rectangular cavity 242. Importantly, see-through holesmay be positioned on any variation of inverted pyramidal cavity shapeand or size in order to maximize the efficiency of the resulting 3-DTFSC.

The mechanical integrities of a reusable template and the released3D-TFSS in this invention are directly related to the quality of theedge definition of the patterns. In order to minimize the cracks thatare initiated from the template and 3D-TFSS edges, encompassingV-grooves are etched into the template at the peripherals to fullyenclose the pyramid area. The encompassing V-grooves are not shown inthe template and 3-D TFSS/TFSC depictions in this disclosure. The widthand depth dimensions of the V-grooves are in the similar range as thepyramid structures.

FIGS. 9 thru 13 are process flows outlining several embodiments forfabricating a 3-D TFSS in accordance with the disclosed subject matter.While the process flows are outlined for fabricating semiconductor-based3-D TFSCs, the overall concepts and methodologies may be extended andapplied to other homojunction and heterojunction semiconductor materials(such as amorphous semiconductor, multi-crystalline semiconductor,polycrystalline semiconductor, CIGS, etc).

The 3-D TFSC fabrication process consists of the following major steps:

-   -   (1) Re-usable template fabrication: 3-D pyramid        patterns/structures are etched from a semiconductor wafer front        surface and into the bulb semiconductor. The structured        semiconductor templates are then used in the formation of 3-D        TFSSs. The templates are capable of being used numerous times to        fabricate numerous 3-D TFSS before being reconditioned or        recycled. The template may be reused for as long as it remains        relatively free of dislocations and/or for as long as it        maintains an acceptable pyramid pattern with its sizes and        surface conditions within acceptable control limits (e.g. as        gauged by in-line metrology);    -   (2) 3-D TFSS (substrate) fabrication: After forming a low and a        high porosity porous semiconductor layers For a graded porosity        porous semiconductor layer) on the template front surfaces        epitaxial semiconductor is grown from the front template        surface. In-situ emitter and base doping may be conducted during        epitaxial semiconductor growth. The 3-D TFSS is formed by        releasing/separating the epitaxial semiconductor layer from the        template. The released 3-D TSFF then goes to subsequent cell        processing steps and the template will be re-used after proper        cleaning and optional reconditioning;    -   (3) Cell fabrication process: In the process module, the 3-D        TFSS is optionally doped to form emitter and base regions if the        doping is not conducted prior to the 3-D TFSS release. Then        surface passivation layer and antireflection layer depositions,        optional contact openings and metallization steps conducted to        form a 3-D TFSC; and    -   (4) Module assembly and integration process: After proper        testing and inspection, the fabricated 3-D TFSC are then        optionally mechanically reinforced, interconnected, encapsulated        and mount in the panels.

The see-through holes are preferably made after the release of the 3-DTFSS and before the surface passivation step by using the openingpatterning and etching methods in the present invention. Alternatively,the see-through-hole making process could be conducted after in otherprocess modules, such as before or after the metallization steps.However, the preferred approach is to create the see-through holes afterthe substrate release and before the surface passivation and cellmetallization processes.

FIG. 9 is a process flow outlining the major process steps and optionsfor the fabrication of a re-usable semiconductor template with etched3-D structures in accordance with the disclosed subject matter. Thefabrication process starts from single-crystal (100) semiconductor waferwith polished or non-polished surfaces. The wafer shapes may becircular, square with truncated or rounded corners, and hexagonal. Thewafer, size may be in the range of 100 mm to 500 mm (or larger in size)with thickness in the range of 0.5 mm to a few millimeters. The firstfabrication process involves forming a hard masking layer, such as athermally grown semiconductor dioxide (SiO₂) or LPCVD semiconductornitride (SiN_(x)), on the wafer surface. In one embodiment, the hardmask layer is formed on all the wafer exposed surfaces, and in anotherembodiment, the hard masking layer is formed only on the front wafersurface where the patterning are to be made. The hard masking layerthickness is between 0.1 um to 1.6 um. In one embodiment, a photoresistpattern is generated on top of the hard mask layer by photolithographysteps including spin-coating (or spray-coating), baking, alignedexposure and developing. In another embodiment, a soft mask layer, withdefined patterns is screen-printed on top of the hard mask layer forreduced fabrication costs. In both embodiments, the patterns are alignedto the wafer (100) direction on the surface of the wafer. In the nextstep, the exposed hard mask layer is chemically etched. For example, thephotoresist pattern is transferred into the oxide layer by etching theexposed oxide with buffered HF solution. The oxide etching step mayrequire using a mechanical fixture for protecting the wafer edge andback surfaces from being etched. Alternatively, the etch process can beperformed in a setup allowing only the front wafer surface to be exposedto the hard mask etching chemical. After that the remaining photoresistlayer is removed by plasma ashing or wet photoresist stripping. Thepatterned semiconductor dioxide layer is used as a hard mask layerduring anisotropic semiconductor etching.

In the next step, anisotropic semiconductor etching, such as KOH(potassium hydroxide) or TMAH (tetramethylammonium hydroxide) solutions,is used to etch the exposed semiconductor area. Anisotropic wet etchingof crystalline semiconductor is one of the key technologies forsemiconductor micromachining. Due to differing chemical reactivity ofcertain crystal planes of the semiconductor, anisotropic etchants etchmuch faster in one direction than in another, exposing the slowestetching crystal planes over time. As an example, when etching a (100)semiconductor wafer with patterned hard mask particularly oriented, ananisotropic etching slow down, markedly at (111) planes ofsemiconductor, relative to their etch rates for other planes. As aresult, the etching exhibits perfectly flat surfaces and well-definedangles. The typical etch temperature is in the range of 50° C. to 80° C.and etch rate is about 0.2 um/min to 1 um/min. The KOH or TMAHsemiconductor etch rate depends upon the orientations to crystallinesemiconductor planes. The (111) family of crystallographic planes areetched at a very slow rate and are normally “stop” planes for theanisotropic etching of a (100) semiconductor wafer with patterned hardmask. As a result, the intersection of two (111) planes or a (111) planewith a bottom (100) plane produce anisotropic etching structures for(100) semiconductor wafers after a time-controlled etch. Examples ofthese structures include V-grooves and pyramidal cavities with sharp tipcavity bottom (where (111) planes meet) or a small flat cavity bottom (aremaining (100) plane). During the semiconductor etching, the waferbackside and edge surfaces are fully protected by un-patterned oxidelayer. The KOH etch may be timely controlled so that a certain invertedpyramidal cavity depth may be reached. Alternatively, the KOH etchingmay be self-terminated when the (111) walls forming the invertedpyramidal cavity meet at the cavity bottom/apex. After the KOH etching,remaining oxide layers are thinner than before the etching because theoxide is also etched in the KOH or TMAH solution, but with a much sloweretch rate than the semiconductor etch.

One of key advantages of using anisotropic wet semiconductor etching,such as Potassium Hydroxide (KOH), is its repeatability and uniformityin semiconductor etching while maintaining a low production cost. Inaddition, when the KOH etching reaches (111) crystallographic planes, itetches the (111) planes with substantially slower etch rates. As aresult, the manufacturing etching process is convenient to control withmuch wider process control windows and much lower cost than othersemiconductor etching methods, such as DRIE semiconductor dry etching.Examples of anisotropic etchants include KOH, NaOH, TMAH, and EDP. Inthe case that wafer edge and backside surfaces are also covered withhard mask layer, the wafers may be etched in a batch semiconductoretching solution. However, in the case the wafer edge or backside areexposed without hard mask layer, the wafer can only be etched in a setupthat only allows the front side of the wafer to be exposed to thesemiconductor etching solution. After the semiconductor etching, theremaining hard mask layer is removed by chemical etching, such as in anHF solution for oxide removal. Next, the etched semiconductor wafers arecleaned in standard wafer cleaning processes, such as RCA1 and RCA2cleanings. Now, the semiconductor template is made and ready for makingthe 3-D TFSS in the following process flows.

In another embodiment of making the semiconductor template, direct lasermicromachining is used to spatially and selectively to remove the hardmask layer. By doing so, the need of soft-mask patterning(photolithography or screen printing) is eliminated. After the laserablation, anisotropic semiconductor etching such as KOH etching is usedto form pyramid structure as described.

In yet another embodiment of making the semi conductor template, directlaser micromachining is used. Laser beam are programmed to scan acrossthe wafers surface and create the periodic patterns with certain depth.After that the semiconductor debris and heat affected zones are etchedin an anisotropic semiconductor etching solution (such as KOH) after aquick etch in a HF solution to remove the thin oxide layer on the laserprocessed areas. Since the debris and heat affected zones have a higheretch rate in anisotropic etch solution than crystallographicsemiconductor, after a time-controlled etched, the inverted pyramidstructures of the disclosed subject matter may be realized. In thismethod, using hard and soft masking layers may be avoided for reducingthe fabrication costs.

FIG. 10 outlines one embodiment of the 3-D TFSS fabrication processflow, in which the emitter and optional base doping is done in-situduring epitaxial semiconductor growth. The fabrication process startswith forming a bi-layer (or graded porosity) porous layer on the frontstructured surface of a reusable semiconductor template. The poroussemiconductor layer is formed by electrochemical HF etching and is used,as a sacrificial layer for epitaxial semiconductor layer release. Theporous semiconductor layer preferably consists of two thin layers withdifferent porosities. The first thin porous semiconductor layer is ontop and is first formed from the bulk semiconductor wafer. The firstthin layer preferably has a lower porosity of 15%˜30%. The second thinporous semiconductor layer is directly grown from the bulb semiconductorand is underneath the first thin layer of porous semiconductor. The thinporous semiconductor layer preferably has a higher porosity in the rangeof 50%˜80%. The top lower porosity layer is used as a crystalline seedlayer high quality epitaxial semiconductor growth and the underneathhigher porosity porous semiconductor layer is used for the easy releaseof the TFSS from the template due to its less dense physical connections(between the epitaxial and bulb semiconductor interfaces) and its weakermechanical strength. Alternatively, a porous semiconductor release layerwith a progressively increased porosity from top to bottom may also beused. In this case, the top portion of the porous semiconductor layerhas a low porosity of 15% to 30% and the lower portion of the poroussemiconductor layer has a high porosity of 50% to 80%. Before theepitaxial semiconductor growth, the wafer is baked in a high temperature(at 950° C. to 1150° C.) hydrogen environment within the epitaxialsemiconductor deposition reactor in order to form coalesced structures(with relatively large voids) within the higher-porosity buried poroussemiconductor layer while forming a continuous surface seed layer ofcrystalline semiconductor on the top of the lower-porosity poroussemiconductor layer. Next, an in-situ-doped mono-crystallinesemiconductor epitaxial layer is deposited on the frontside only. Thebulk base of the epitaxial layer is p-type, boron (B₂H₆) doped, whilethe lower base region (initial epitaxial growth at bottoms) may beheavily p-type doped and emitter region (end epitaxial growth at top) isn-type, phosphorus (PH₃) doped. In an alternative method, only the baseis in-situ-doped, while the emitter doping may be conducted after therelease of TFSS by a variety of methods, such as diffusion,ion-implantation, and alloying with aluminum contact. Not only does thein-situ-doped epitaxial growth provide the most convenient creation ofemitter and base doping, but also allows the active tuning/controllingof the doping type and concentration across the epitaxial layerthickness. The thickness of the epitaxial layer is in the range of 1 umto 50 um. Next, an optional thin oxidation step is conducted to grow aless than 100 nm thick oxide on the top epitaxial semiconductor surface.The thin oxide layer could be used as a sacrificial hard masking layerto etch the remaining porous semiconductor layer and quasi-mono-crystalline-semiconductor (QMS) layer after the 3-D TFSS release. In the nextstep, prior to the release, a border definition trench is made on theperipheral of the active wafer area, to facilitate the release.

Detailed methods of making the border definition trenches include adouble-trench method. An inner shallow trench defines the outer boundaryof the finished 3-D TFSS. The outer deep trench is cut through poroussilicon layer and provides access to the porous silicon layer so thatthe 3-D TFSS may be released. The encompassing trenches are formed bycontrolled laser cutting and their depths are in the range of 5 um to100 um. The trenches define the boundary of the 3-D TFSS to be releasedand allow initiation of the release from the trenched region. In anotherembodiment, instead of making the laser cut, the wafer edge is polishedby an abrasive tape for removing the epitaxial semiconductor wrap-aroundand exposes the porous semiconductor layers at the wafer edge. In yetanother embodiment, instead of making laser cut trenches on the frontsurface, the wafer or a batch of wafer stacked together could beedge-ground to a controlled depth in order to facilitate the entireepitaxial semiconductor layer to be released from the template. In thenext step, the epitaxial lager of semiconductor is released/separatedfrom the template. U.S. Patent Publication No. 2010-0022074 filed May28, 2009 entitled, SUBSTRATE RELEASE METHODS AND APPARATUS, by thecommon inventors David Xuan-Qi Wang Mehrdad M. Moslehi is herebyincorporated by reference. The above reference patent applicationdiscloses detail methods of releasing the: epitaxial layer to form areleased 3-D TFSS. In one method, the 3-D TFSS is released in anultrasonic DI-water bath. In yet another method, the 3-D TFSS isreleased by direct pulling with wafer backside and top epitaxial vacuumchucked. After removal of the epitaxial semiconductor layer from thetemplate, the template is cleaned by using diluted HF and diluted wetsemiconductor etch solution, such as TMAH and/or KOH to remove theremaining porous semiconductor layers and semiconductor particles. Thenthe template is further cleaned by conventional semiconductor wafercleaning methods, such as RCA1 and/or RCA2 wet cleaning to removalpossible organic and metallic contaminations. Finally, after properrinsing with DI water and drying, the template is ready for anotherre-use cycle. Next, the released TFSS backside surface is cleaned byshort semiconductor etching using KOH or TMAH solutions to remove thesemiconductor debris and fully or partially remove the QMS layer.

In the case of making a see-through 3-D TFSS, one embodiment of thedisclosed subject matter is to etch the see-through openings from thesmall pyramids according to a self-aligned process that includes hardmask layer and/or soft mask layer coatings, controlled anisotropicsemiconductor etching and other process steps. In another see-throughholes making process, the openings are etched from the large pyramidsand the process also includes self-aligned coating, mechanical lappingand etching steps. In yet another, see-through hole making process, theopenings are made from flat area by laser drilling or coating,photolithography and controlled, semiconductor etching. The detailembodiments of the see-through hole making processes are disclosed inthe later sections of the present application.

After forming the see-through holes, the remaining mask layer is removedby chemical etching and the see-through 3-D TFSS is cleaned by standardRCA1 and RCA2 cleaning following by DI water rinsing and drying. Thecleaning of the 3-D TFSS with or without see-through holes may requireproper fixtures to protect the TFSS from mechanical damages duringcleaning and drying. The cleaned 3-D TFSS with or without see-throughholes, is ready for the subsequent ceil process.

FIG. 11 outlines an embodiment of the process flow for fabrication ofthe 3-D TFSSs with or without see-through holes. In this process flow,the emitter and BSF doping is conducted after the epitaxialsemiconductor growth process but before the 3-D TFSS release. Theprocess of forming the bi-layer porous semiconductor is same asdescribed in the above section for FIG. 10. After forming the poroussemiconductor layers and subsequent hydrogen annealing, epitaxialsemiconductor is grown on top of the porous semiconductor layers. Inthis case, there is no particular emitter and BSF doping process to beconducted except a uniform or graded p-type, boron doping to provide ap-type epitaxial semiconductor layer with a thickness of 1 um to 60 um.Note that all the doping polarities for emitter and base may beinverted, (i.e., p-type emitter and n-type base). After the epitaxialsemiconductor growth and before the 3-D TFSS release, the emitter dopingis conducted by phosphorus diffusion in furnace equipment by using POCl3phosphorus doping source. After the diffusion process, HF solution basedchemical etching is used to remove the phosphorus glass on the topsurface in a deglazing process. In another embodiment of emitter doping,ion implantation of n-type doping source can be conducted followed byannealing. In yet another embodiment, liquid or paste n-type dopingsource can be coated and cured on the top portions (ridges) of theepitaxial semiconductor layer by self-aligned spatially selectivecoating, such as roller coating, screen printing or dip coating. Afterdrying/curing (250° C. to 400° C.) and thermal annealing (800° C. to950° C.), the remaining dried doping source layer is removed by chemicaletching, such as in a HF solution. The roller coating, screen printingand dip coating methods axe types of so-called “Liquid Transfer Coating”(LTC). Examples of LTC methods are disclosed in U.S. Patent PublicationNo. 2009-0042320, entitled, METHODS FOR LIQUID TRANSFER COATING OFTHREE-DIMENSIONAL SUBSTRATES, by common inventors David Xuan-Qi Wang andMehrdad M. Moslehi is hereby incorporated by reference. Other methodsmay also be used for this spatially selective etch liquid/paste coating.It is to be noted, that the methods of using ion implantation andliquid/paste selective coating of the doping source for emitter dopingcould provide selective emitters that are heavily doped. In other words,on the front side, the top ridge region is doped with n++ and pyramidsidewall areas are doped with n+. The selective emitter doping effectscan provide higher cell efficiencies than non selective emitter cells.In the next step, an optional thin oxidation step is conducted to growan oxide layer less than 100 nm thick on the top epitaxial semiconductorsurface. The thin oxide layer could be used as a hard masking layer toetch the remaining porous semiconductor layer and QMS layer after the3-D TFSS release. Next, as described above, border trenches are cut intothe semiconductor surface by laser cutting. Alternatively, the waferedge is polished by an abrasive tape for removing the epitaxialsemiconductor wrap-around and exposes the porous semiconductor layers atthe wafer edge. Then the 3-D TFSS is released from the template byeither ultrasonic DT water bath or direct mechanical pulling with vacuumchucking from both sides. The semiconductor template is then cleaned byshort semiconductor etching using KOH or TMAH solutions to remove thesemiconductor debris and restore the good quality template surface andprofile. After the standard RCA1 and RCA2 chemical cleaning, DI waterrinsing and drying, the template is sent for the next reuse cycle. Next,the released TFSS backside surface is cleaned by short semiconductoretching using KOH or TMAH solutions to remove the semiconductor debrisand fully or partially remove the QMS layer. Another optional thinoxidation process is then conducted and the backside oxide layer is thenremoved to expose the TFSS backside semiconductor surface.

Shown, there are three options/embodiments for performing BSF p-typedoping. One embodiment is achieved by furnace equipment based diffusionof p-type (boron) doping. After the diffusion, HF solution basedchemical etching is used to remove the thin glass layer on the backsurface in a deglazing process. In another embodiment of BSF doping, ionimplantation of p-type doping source can be conducted following byannealing. In yet another embodiment, liquid p-type doping source can becoated and cured on the backside surface by self-aligned spatiallyselective coating, such as roller coating, dip coating or screenprinting.

After drying/curing (250° C. to 400° C.) and thermal annealing (800° C.to 950° C.), the remaining dried doping source layer is removed bychemical etching such as in a HF solution. The roller coating, screenprinting and dip courting methods are types of so-called “LiquidTransfer Coating” (LTC). Examples of LTC methods are disclosed in U.S.Patent Publication No. 2009-0042310, entitled, METHODS FOR LIQUIDTRANSFER COATING OF THREE-DIMENSIONAL SUBSTRATES, by common inventorsDavid Xuan-Qi Wang and Mehrdad M. Moslehi which is incorporated byreference. Other methods may also be used for this spatially selectiveetch liquid/paste coating. It is to be noted that the methods of usingion implantation and liquid/paste selective coating of the doping sourcefor BSF doping may provide selective BSF that are heavily doped. Inother words, on the back side, the pyramid tip region is doped with p++and pyramid sidewall areas are doped with p+ back surface field (BSF).After forming the emitter and optional back surface field base regions,the 3-D TFSS is ready for downstream cell process.

In the case of making a see-through 3-D TFSS, as described for FIG. 10,the see-through openings are etched from the small pyramids in aself-aligned process that includes hard mask layer and/or soft mashlayer coatings, controlled anisotropic semiconductor etching and otherprocess steps. In another see-through holes making embodiment, theopenings are etched from the large pyramids and the process alsoincludes self-aligned coating, mechanical lapping and etching steps. Inyet another see-through hole making embodiment, the openings are madefrom flat areas by laser drilling or coating, photolithography andcontrolled semiconductor etching. The detailed embodiments of thesee-through hole making processes are disclosed in the later sections ofthe present application.

After forming the see-through holes, the remaining mask layer is removedby chemical etching and the see-through 3-D TFSS it cleaned by standardRCA1 and RCA2 cleaning following by DI water rinsing and drying. Thecleaning of the 3-D TFSS with or without see-through holes may requireproper fixtures to protect the TFSS from mechanical damages duringcleaning and drying. The cleaned 3-D TFSS with or without see-throughholes, is ready for the subsequent cell process.

FIG. 12 outlines yen another embodiment of a process flow forfabrication of the 3-D TFSSs with or without see-through holes. In thisembodiment, the emitter and BSF doping is conducted after the 3-D TFSSrelease. The process of forming the bi-layer porous semiconductor issame as described in the above section for FIG. 10. After forming theporous semiconductor layers and subsequent hydrogen anneal lug,epitaxial semiconductor is grown on top of the porous semiconductorlayers. In this case, there is no particular emitter and BSF dopingprocess to be conducted except an optional uniform low level p-type,boron doping to provide a re-type epitaxial semiconductor layer with athickness of 1 um to 50 um. In the next step, an optional, thinoxidation step is conducted, to grow a less than 100 nm thick oxide onthe top epitaxial semiconductor surface. The thin oxide layer could beused as a hard masking layer to etch the remaining porous semiconductorlayer and QMS layer after the 3-D TFSS release. Next, as described inthe earlier, border trenches are cut into the semiconductor surface bylaser cutting. Alternatively, the wafer edge may be polished by anabrasive tape for removing the epitaxial semiconductor wrap-around andexposes the porous semiconductor layers at the wafer edge. Then the 3-DTFSS is released from the template by either ultrasonic DI water bath ordirect mechanical pulling with vacuum chucking from both sides. Thesemiconductor template is then cleaned by short semiconductor etchingusing KOH or TMAH solutions to remove the semiconductor debris andrestore the good quality template surface and profile. After thestandard RCA1 and RCA2 chemical cleaning, DI water rinsing and drying,the template is sent for the next, reuse cycle. Next, the released TFSSbackside surface is cleaned by short semiconductor etching using KOH orTMAH solutions to remove the semiconductor debris and fully or partiallyremove the QMS layer. Next, a thin oxidation is thermally grown on the3-D TFSS and frontside oxide is then removed to expose the semiconductorsurface for emitter doping.

Shown are three options/embodiments for performing emitter n-typedoping. In the first embodiment, the emitter doping is conducted byphosphorus diffusion, in furnace equipment by using POCl3 phosphorusdoping source. After the diffusion process, HF solution based chemicaletching is used to remove the phosphorus glass on the top surface in adeglazing process. In another embodiment of emitter doping, ionimplantation of n-type doping source can be conducted following byannealing. In yet another embodiment, liquid/paste n-type doping sourcecan be coated and cured on the top portions (ridges) of the epitaxialsemiconductor layer by self-aligned spatially selective coating, such asroller coating, screen printing or dip coating. After drying/curing(2500° C. to 400° C.) and thermal annealing (800° C. to 950° C.), theremaining dried doping source layer is removed by chemical etching suchas in an. HE solution. The roller costing, screen printing and clipcoating methods are types of so-called “Liquid Transfer Coating” (LTC).Examples of LTC methods are disclosed in U.S. Patent Publication No.2009-0042320, entitled, METHODS FOR LIQUID TRANSFER COATING OFTHREE-DIMENSIONAL SUBSTRATES, by David Xuan-Qi Wang and Mehrdad M.Moslehi is hereby incorporated by reference. Other may also be used forthis spatially selective etch liquid/paste coating. It is to be notedthat the methods using ion implantation and liquid/paste selectivecoating of the doping source for emitter doping may provide selectiveemitters that provide heavily doped emitter contact regions. In otherwords, on the front side, the top ridge region is doped with heavier n++doping and pyramid sidewall areas are doped with lighter n+ doping.Next, before optional base back surface field doping, another thin layerof oxide is thermally grown on the TFSS and the backside oxide is thenremoved to expose the backside semiconductor surface.

Shown are three options/embodiments for performing back surface field(BSF) p-type doping. One process uses furnace equipment based diffusionof p-type (boron) doping. After the diffusion, HF solution basedchemical etching is used to remove the thin glass layer on the backsurface in a deglazing process. In another process embodiment of BSFdoping, ion implantation of p-type doping source can be conductedfollowing by annealing. In yet another embodiment, liquid/paste p-typedoping source can be coated and cured on the backside surface byself-aligned spatially selective courting, such as roller coating, dipcoating or screen printing. After drying/curing (250° C. to 400° C.) andthermal annealing (800° C. to 950° C.), the remaining dried dopingsource layer is removed by chemical etching such is in an HF solution.The roller coating, screen printing and dip coating methods are types ofso-called LTC. Other methods may also be used for this spatiallyselective etch liquid/paste coating. It is to be noted that the methodsof using ion implantation and liquid/paste selective coating of thedoping source for back surface field (BSF) doping may provide selectivebases that are heavily doped. In other words, on the back side, thepyramid, tip region is doped with p++ (for reduced base contactresistance) and pyramid, sidewall areas are doped with p+.

After forming the emitter and BSF regions, the 3-D TFSS is ready fordownstream cell process. In the case of making a see-through 3-D TFSS,same as described in FIG. 10, one embodiment is to etch the see-throughopenings from the small pyramids using a self-aligned process thatincludes hard mask layer and/or soft mask layer coatings, controlledanisotropic semiconductor etching and other process steps. In anothersee-through holes making embodiment, the openings are etched from thelarge pyramids and process also includes self-aligned coating,mechanical lapping and etching steps. In yet another see-through holemaking embodiment, the openings are made from flat area by laserdrilling or coating, photolithography and controlled semiconductoretching. The detail embodiments of the see-through hole making processesare disclosed in the later sections of the present application.

After forming the see-through holes, the remaining mask layer is removedfey chemical, etching and the see-through 3-D TFSS is cleaned bystandard RCA1 and RCA2 cleaning following by DI water rinsing anddrying. The cleaning of the 3-D TFSS with or without see-through holesmay require proper fixtures to protect the TFSS from mechanical damagesduring cleaning and drying. The cleaned 3-D TFSS with or withoutsee-through holes, is ready for the subsequent cell process.

FIG. 13 outlines a process flow for the fabrication a 3-D TFSC with orwithout see-through holes. The process steps include surface passivationon front and back surfaces of the released 3-D TFSS, anti-reflectionlayer coating, selective and local dielectric layer opening, emitter andbase metal plating, and back reflector metal deposition.

After surface cleaning of the 3-D TFSS, the process starts with surfacepassivation. As an example, a thin semiconductor oxide (SiO₂) of 3 nm to100 nm thick is thermally grown by oxidation in the temperature range of800° C. to 950° C. to serve as the passivation layer for both sides.Next, a 3 nm to 100 nm thick PECVD semiconductor nitride (SiN_(x)) isdeposited on the front side oxide surface for serving as ananti-reflection (ARC) layer and another front surface passivation layer.As an alternative to or in addition to the thin thermal oxide, a thinlayer of reactive sputtered or PECVD deposited aluminum oxide (AlO_(x))or PECVD SiN_(x) may be used for the back surface passivation. Next, theemitter and base contact areas are subsequently opened by spatiallyselective etching of dielectric layer(s). In one embodiment, dielectricetching liquid, such as diluted HF, is roller coated or dip coated tothe front ridge or the back pyramid tip regions for direct etching thedielectric layers to be opened. After the etching, the 3-D TFSS isrinsed with DI water and dried. In another embodiment, the dielectricetching paste is screen printed on the front ridge or the back pyramidtip regions followed by a heated curing step. During the heating, thedielectric layer is etched by the coated etch paste.

After the etching, the remaining etch paste is strip and the 3-D TFSS iscleaned. The roller coating, screen printing and dip coating methods aretypes of so-called LTC. Other methods may also be used for thisspatially selective etch liquid/paste coating. In yet anotherembodiment, controlled laser ablation is used to selectively remove thedielectric layers. In order to effectively remove the dielectric layerwithout or minimum damage to the underneath semiconductor surface,short-pulsed (Pico-second) UV laser is preferred. In yet anotherembodiment, photolithography patterning of the emitter and base contactopening could be conducted. In this method, photoresist is sprayed anddried following by aligned exposure and resist developing. Upon creatingthe contact opening pattern by photoresist, chemical etching of theexposed dielectric layers may be conducted. Then the remaining resistlayer is removed. The above methods of spatially selective etching ofdielectric layers may be used for both emitter and base contactopenings; however the etching source material may be different. Afterthe emitter and base contact openings process, an optional aluminum (Al)deposition by evaporation or sputtering is conducted on the TFSSbackside followed by an annealing/fire-through at 800° C. to 950° C. Theannealing process provide Al-Si alloy and form p++ region at basecontact. This p++ a base contact is effective in the case that boron p++region is not perforated by the earlier diffusion process. The remainingaluminum at TFSS backside can be optionally removed. Next, the emitterand base metallization is conducted by selective electroplating orelectroless plating of nickel (Ni) in the thickness range of 50 nm to300 nm following by optional Ni sintering. Next, electroplating orelectroless plating of Ag is conducted on both the emitter and basecontact regions. The Ni and Ag plating may be replaced by Cu platingwith a suitable barrier layer (e.g. Ta). Next, an optional forming gasannealing (FGA) is performed at temperature range of 200° C. to 400° C.to reduce the interconnect resistance and help with surface/bulkpassivation of metals. Next, a thin layer of Al is PVD, evaporation, orsputtered on the TFSC backside to serves as a back-reflection mirror.Optionally, the FGA process could be performed as this stage. In thenext step, the fabricated 3-D TFSC is packaged in a solar module orpanel.

FIG. 14 outlines another process flow for fabrication of a 3-D TFSC withor without see-through holes. The process steps include surfacepassivation on front and back surfaces of the released 3-D TFSS,anti-reflection layer coating, screen printed emitter and base metalcontacts, and back reflector metal deposition. After surface cleaning ofthe 3-D TFSS, the process starts with surface passivation. As anexample, a thin semi conductor oxide (SiO₂) or 3 nm to 10 nm thick isthermally grown by oxidation in the temperature range of 800° C. to 950°C. to serve as the passivation, layer for both sides. Next, a 3 nm to100 nm thick PECVD semiconductor nitride (SiN_(x)) is deposited on thefront side oxide surface for serving as an anti-reflection (ARC) layerand another front surface passivation layer. Alternative or in additionto the thin thermal oxide, a thin layer of reactive sputtered or PECVDdeposited aluminum oxide (AlO_(x)) or PECVD SiN_(x) can be used for theback surface passivation. Backside (base) metallization is performed bycoating metal paste or like to the backside pyramid tip regions. In thecase that p++ base contact region is formed. In the upstream diffusionprocess, Al paste could be used as the base contact metal. In the casethat base contact region, has not been previously formed, Al ispreferred to be the base metal since after annealing it may form a p++base contact region from the Al—Si alloy. The Ag or Al paste can beroller coated, dip coated or screen printed in a self-aligned coatingprocess, i.e. only the pyramid tip regions are coated. Afterdrying/curing of the coated metal paste by IR or oven heating, the basemetal fire-through process may be optionally performed so that, the Alcan be fired through the backside dielectric layer(s) and form directmetal to semiconductor base contact.

Next, emitter metallization coating is performed in two optional paths.In the first option, metal paste is roller-coated, dip-coated orscreen-printed to the top ridge portions of the front surface in aself-aligned manner. Again, other liquid transfer coating methods, suchas those disclosed in U.S. Patent Publication No. 2009-0042320,entitled, METHODS FOR LIQUID TRANSFER COATING OF THREE-DIMENSIONALSUBSTRATES, by David Xuan-Qi Wang and Mehrdad M. Moslehi is herebyincorporated by reference, may also be used for the base and emittermetal paste coating process. In the second, option, the metal, paste isscreen-printed with alignment to certain features on the 3-D TFSS, andresults with only specific area, such as stripes of flat areas, coatedwith the metal paste. Next, the coated emitter metal paste is dried andfired through the front side surface passivation and ARC dielectriclayers to form emitter contact. Both the emitter silver and basealuminum may be co-fired using a single thermal anneal step instead oftwo separate firing steps. After the front and back metal fire-throughprocess, an optional electroplating or electroless plating of an Aglayer may be conducted for increased emitter and base electricalconductivity. The Ni and Ag plating may be replaced by Cu plating with asuitable barrier layer (e.g. Ta). Next, a thin layer of Al is depositedby evaporation or sputtering on the TFSC backside to serve as aback-reflection mirror. Next, an optional forming gas annealing (FGA) isperformed at temperature range of 200° C. to 400° C. to reduce theinterconnect resistance and help with surface/bulk passivation ofmetals. In the next step, the fabricated 3-D TFSC is packaged in a solarmodule or panel.

FIG. 15A through 15D illustrate schematic cross-sectional drawings of atemplate fabrication process. A mono-crystalline (100) semiconductorwafer of 100 mm to 300 mm is used for making the template. The thicknessof the wafers is in the range of 0.5 mm to 2 mm. The template makingprocess may also be applied onto a polished as well as non-polishedsurface. Alternatively, wafers with square or quasi-square shapes couldalso be used.

As shown in FIG. 15A, a thermally grown oxide layer of semiconductoroxide 252, 0.5 um to 1.5 um thick, is used as the hard mask layer on(100) semiconductor wafer 250. The oxide on the wafer edge is not shown.Next, photolithographic defined or screen-printed photoresist pattern254 is applied on the front wafer surface of (100) semiconductor wafer250. The photolithography process includes photoresist coating, baking,exposure, developing and post baking. It is important the square-openingpatterns are precisely aligned, to the wafer <100> direction on thefront surface. Next, the photoresist pattern is transfer red to thefront oxide layer, as shown in FIG. 15B, by controlled, oxide etching ina buffered HF solution. After stripping off the remaining photoresistlayer and wafer clearing, the front oxide patterned wafer is etched inanisotropic semiconductor etch solution, such as KOH and TMAH. Theetching temperature is in the range of 50° C. to 80° C.

As shown in FIG. 15C, the semiconductor wafer 250 backside and edgesurfaces are fully protected by the un-patterned oxide layer during theKOH etching. The KOH etch can be timely controlled so that a certainpyramid cavity depth could be reached creating cavity surface 256aligned along the (100) crystallographic plane. Alternatively, the KOHetching could be self-terminated when the four pyramid (111) sidewallsmoot at the cavity bottom. Shown, sidewalls 258 are aligned along the(111) crystallographic plane. After the KOH etching, the remaining oxidelayer is removed in diluted HF solution followed by standard wafercleaning in RCA1 and RCA2 with DI water rinsing and drying.

FIG. 15D illustrates a made semiconductor template with mainly (111)semiconductor crystallographic planes on its front surface. And theangle between the cavity sidewalls to the lateral plane is 54.7°. Theadvantages of the template include (i) the template KOH etching processis more convenient to control and with low cost in manufacturingcompared to other semi conductor etching methods, such as RIE dryetching; (ii) The (111) plane dominant 3-D structure provides goodporous semiconductor forming uniformity and epitaxial semiconductorquality due to their sidewall smoothness and predictable and repeatableepitaxial growth rates; (iii) The dimensions, shape and profiles of theinverted pyramid structure on the template can be maintainedconveniently and restored easily by a short KOH etching if needed aftermultiple template reuse cycles.

FIGS. 16A through 16C are cross-sectional drawings illustrating a 3-DTFSS fabrication process with in-situ emitter, base, and BSF doping. Asshown in FIG. 16A, porous semiconductor layer 282 is formed byelectrochemical HF etching on the front surfaces of template 280 and itis used as a sacrificial layer for epitaxial semiconductor layerrelease. The porous semiconductor layer preferably consists of two thinlayers with different porosities. The first thin porous semiconductorlayer is on top and is first formed from the bulk semiconductor wafer.The first thin layer preferably has a lower porosity of 15%-30%. Thesecond thin porous semiconductor layer is directly grown from the bulksemiconductor and is underneath the first thin layer of poroussemiconductor. The 2^(nd) thin porous semiconductor layer preferably hasa higher porosity in the range of 50%-80%. The top lower porosity layeris used as a crystalline seed layer for high quality epitaxialsemiconductor growth and the underneath higher porosity poroussemiconductor layer is used for easier TFSS releasing doe to its lessdensity physical connections (between the epitaxial and bulksemiconductor interfaces) and its weaker mechanical strength.Alternatively, a single porosity release layer with a progressivelyincreased porosity from top to bottom can also be used. In this case,the top portion of the porous semiconductor layer has a low porosity of15% to 30% and the lower portion, of the porous semiconductor layer hasa high porosity of 50% to 80%. Before the epitaxial semiconductorgrowth, the wafer is baked in a high temperature (at 950° C. to 1150°C.) hydrogen environment within the epitaxial semiconductor depositionreactor in order to form coalesced structures (with relatively largevoids) within the higher-porosity buried porous semiconductor layerwhile forming a continuous surface seed layer of crystallinesemiconductor on the top of the lower-porosity porous semiconductorlayer.

As shown in FIG. 16B, in-situ-doped mono-crystalline semiconductorepitaxial layer 284 is deposited on the front side only of template 280.The bulk base of the epitaxial layer is p-type, boron (B₂H₆) doped,while the back surface field (BSF) region 288 (initial epitaxial growthat bottom) may be optionally heavily p-type doped and emitter region 286(end epitaxial growth at top) is n-type, phosphorus (PH₃) doped. Thethickness of the epitaxial layer is in the range of 5 um to 60 um.

In the next step, prior to the release, an encompassing border trenchmay be made on the peripheral of the active wafer area to facilitate therelease.

Detailed methods of making the border definition trenches include acurable-trench method. An inner shallow trench defines the outerboundary of the finished 3-D TFSS. The outer deep trench is cut throughporous silicon layer and provides access to the porous silicon layer sothat the 3-D TFSS may be released. The encompassing trenches are formedby controlled laser cutting and their depths are in the range of 5 um to100 um. The trenches define the boundary of the 3-D TFSS to be releasedand allow initiation of the release from the trenched region. In anotherembodiment, instead of making tine laser cut, the wafer edge is polishedby an abrasive tape for removing the epitaxial semiconductor wrap-aroundand exposes the porous semiconductor layers at the wafer edge.

The encompassing trenches are formed by controlled laser cutting andtheir depths are in the range of 5 um to 100 um. The trenches define theboundary of the 3-D TFSS to be released and allow initiation of therelease from the trenched region. In another embodiment, instead ofmaking the laser cut, the wafer edge is polished by an abrasive tape forremoving the epitaxial semiconductor wrap-around and exposes the poroussemiconductor layers at the wafer edge. In yet another embodiment,instead of making laser cut trenches on the front surface, the wafer ora batch of wafer stacked together could be edge-ground to a controlleddepth, in order to facilitate the entire epitaxial semiconductor layerto be released from the template.

In the next step, the epitaxial layer of semiconductor isreleased/separated from the template. The release methods disclosed inU.S. Patent Publication No. 2010-0022074 filed May 28, 2009 entitled,SUBSTRATE RELEASE METHODS AND APPARATUS, by the common inventors DavidXuan-Qi Wang Mehrdad M. Moslehi are hereby incorporated by reference. Inone method, the 3-D TFSS is released in an ultrasonic DI-water bath. Inyet method, the 3-D TFSS is released by direct pulling with waferbackside and top epitaxial vacuum chucked. After removal of theepitaxial semiconductor layer from the template, the template is cleanedby using diluted HF and diluted wet semiconductor etch solution, such asTMAH and/or KOH to remove the remaining porous semiconductor layers andsemiconductor particles. Then the template is further cleaned byconventional semiconductor wafer cleaning methods, such as RCA1 and/orRCA2 wet cleaning to removal possible organic and metalliccontaminations. Finally, after proper rinsing with DI water and drying,the template is ready for another re-use cycle. Next, the released TFSSbackside surface is cleaned by short semiconductor etching using KOH orTMAH solutions to remove the semiconductor debris and fully or partiallyremove the QMS layer. FIG. 16C illustrates cleaned reusable template 280and the released and cleaned 3-D TFSS comprised of semiconductorepitaxial layer 284, in-situ n-type doped emitter layer 286, and in-situp-type BSF 288.

FIGS. 17A through 17C are cross-sectional drawings illustrating a 3-DTFSS fabrication process in which neither the emitter nor BSF isparticularly doped during epitaxial growth. However, the bulk base ofepitaxial layer 304 is in-situ p-type, boron (B₂H₆) doped. FIG. 17Aillustrates porous bi-layer 302 formation process on template 300 whichis same as described in the above section for FIG. 16A. The emitter 306doping is conducted by phosphorus diffusion or ion implantation afterepitaxial semiconductor 304 growth as shown in FIG. 17B. The post ionimplantation annealing may be performed before or after the 3-D TFSSrelease and the optional base p-type doping may be conducted after therelease of 3-D TFSS. FIG. 17C illustrates cleaned and reusable template300 and released and cleaned 3-D TFSS comprising epitaxial semiconductorlayer 304 and n-type doped emitter 306.

FIGS. 18A through 18C are cross-sectional drawings illustrating a 3-DTFSS fabrication process in which neither the emitter nor back surfacefield (BSF) is particularly doped during epitaxial growth The emitterand BFS doping processes are conducted after the release of 3-D TFSS.FIG. 18A illustrates porous bi-layer 312 formation process on template310 which is same as described in the above section for FIG. 16A. Asshown in FIG. 18B, epitaxial semiconductor layer 314 is not particularlydoped for emitter or BSF, however, the bulk base p-type doping isperformed in-situ to form a p-type epitaxial bulk base layer. FIG. 18Cillustrates cleaned reusable template 310 and the released and cleaned3-D TFSS comprised of p-type doped epitaxial semiconductor layer 314.

FIGS. 19A through 19C illustrates a selective emitter and BSF dopingprocess performed on a released 3-D TFSS by liquid transfer coating. Asshown in FIG. 19A, liquid or paste n-type doping source 322 is coatedand cured on the top portions (ridges) of the epitaxial semiconductorlayer 320 by self-aligned spatially selective coating, such as rollercoating, screen printing or dip coating. After drying/curing (250° C. to400° C.) and thermal annealing (800° C. to 950° C.), the remaining drieddoping source layer is removed by chemical etching such is in a HFsolution. The roller coating, screen printing and slip coating methodsare types of so-called liquid transfer coating—such as the LTC methodsdisclosed in U.S. Patent Application Pub. No. 2009-0042320 which isincorporated by reference. Other methods may also be used for thisspatially selective etch liquid/paste coating. The liquid/pasteselective coating of the doping source for emitter doping could provideselective emitters that are heavily doped. In other words, on the frontside, the top ridge region is doped with n++ and pyramid sidewall areasare doped wish n+ (shown as n-type doped emitter 326 in FIG. 19C).

Next, as shown in FIG. 19B, a liquid or paste p-type doping source 324is coated, and cured on the backside surface of epitaxial layer 320 byself-aligned spatially selective coating, such as roller coating, dipcoating or screen printing. After drying/curing (250° C. to 400° C.) andthermal annealing (800° C. to 950° C.), the remaining dried dopingsource layer is removed by chemical etching such is in a HF solution.The roller coating, screen printing and dip coating methods are types ofso-called “Liquid Transfer Coating” (LTC). The examples of LTC foundU.S. Patent Publication No. 2009-0042320, entitled, METHODS FOR LIQUIDTRANSFER COATING OF THREE-DIMENSIONAL SUBSTRATES, by common inventorsDavid Xuan-Qi Wang and Mehrdad M. Moslehi are incorporated by reference.Other methods may also be used for this spatially selective etchliquid/paste coating. The liquid/paste selective coating of the dopingsource for base doping could provide selective Cases that are heavilydoped. In other words, on the back side, the pyramid tip region is dopedwith par and pyramid sidewall areas are doped with p+ (shown as p-typedoped base 328 in FIG. 19C). Alternatively, a liquid p-type dopingsource may be spray coated to the entire backside surface. Afterannealing, a continuous BSF region is formed.

FIG. 20A shows the schematic top view of the 3-D TFSS prior to thesee-through hole etching process, in which the small pyramids will beselectively patterned and etched from their backsides. FIGS. 20B through20D illustrate a method of forming the see-through holes from a released3-D TFSS by etching small pyramidal cavities.

FIG. 20B is a depiction of a cross-section along the C-C dash line inFIG. 20A. In FIG. 20B, front surface mask layer 342 is deposited tocover the entire front surfaces of released 3-D TFSS 340 comprising andepitaxial semiconductor layer, a top emitter doping region, and a bottombase doping region. The mask material is to protect the front surfacefrom the subsequent semiconductor etching, such a KOH etching. The maskmaterials include but not limited to oxide, nitride,evaporated/sputtered chrome (Cr), and polymeric soft mask materials suchas coated PMMA. In FIG. 20B, back surface mask material 344, which maybe different than the front mask material, is selectively coated on thebackside of 3-D TFSS 340. The backside coating coverage is preferablyperformed by self-alignment rather than by photolithography, even thoughboth methods may be used in accordance with the disclosed subjectmatter. The backside selective coating is preferred conducted from aliquid source and by a controlled liquid transfer coating (LTC), whichincludes of roller coating, dip coating, and screen printing. As anexample, in a roller coating method, the coating material only wet thetips of the large pyramids and from there to spread and cover the entiresurfaces of the large pyramids and the grooves between the large andsmall pyramids. The liquid amount that is transferred from the largepyramid tips is limited and precisely controlled so that furtherspreading on the surfaces of small pyramids is prohibited. In addition,the coating liquid temperature, viscosity, solvent properties, and TFSSbackside surface conditions are important factors to affect the coatingcoverage control. After the self-aligned LTC process, the coated maskmaterial is dried and cured followed by a KOH semiconductor etching.Even through the semiconductor surface to be etched are mostly (111)planes, these (111) planes meet and form convex edges and sharp tipswhich are etched in a much faster rates than regular crystallographicplanes. As a result, the exposed semiconductor (111) planes are etchedfrom their peripherals and finally etched through. The see-through holeopening size may be in-situ controlled by using a light source and anoptical detector to measure the light transmissivity during etching andterminated the etching process when a pre-determined opening size/ratio.After the KOH etching, the coating material on both the front andbacksides are stripped and 3-D TFSS 340 with see-through hole 346 iscleaned and dried (as shown in FIG. 20D).

FIG. 21A shows the schematic top view of the 3-D TFSS prior to thesee-through hole etching process, in which the small pyramids will heselectively patterned and etched from their backsides. FIGS. 21B through21D illustrate a method of forming the see-through holes from a released3-D TFSS by etching small pyramidal cavities.

FIG. 21B depiction of a cross-section along the C-C dash line in FIG.21A. In FIG. 21B, a hard mask layer is deposited to cover the frontsurface (hard mask layer 352), back surface (354) and side surfaces of3-D TFSS 350 comprising and epitaxial semiconductor layer, a top emitterdoping region, and a bottom base doping region. The hard mask materialis used to protect surfaces from the subsequent semiconductor etching,such a KOH etching. The mask materials include but not limited tothermally grown oxide and LPCVD nitride.

In FIG. 21C, soft mask material 356 is selectively coated on thebackside of 3-D TFSS 350, The backside coating may be conducted by theself-alignment selective coating or by photolithographic definedphotoresist coating to define the openings at the small pyramid backsurfaces. The backside selective coating is preferably conducted from aliquid source and by a controlled liquid transfer coating (LTC) method,which may include roller coating, dip coating, and screen printing. Asan example, in a roller coating method, the coating material only wetthe tips of the large pyramids and from there to spread and cover theentire surfaces of the large pyramids and the grooves between the largeand small pyramids. The liquid amount that is transferred from the largepyramid tips is limited and precisely controlled so that furtherspreading on the surfaces of small pyramids is prohibited. In addition,the coating liquid temperature, viscosity, solvent properties, and TFSSbackside surface conditions are important factors to affect the coatingcoverage control. After the backside soft mask patterning by eitherself-aligned LTC process or photolithography, the opening pattern of thesoft mask is transfer red onto the hard mask by etching the exposed hardmask. After striping the soft mask layer, KOH semiconductor etching isused to form the openings on the small pyramid by the backside etching.Even thorough the semiconductor surface to be etched are most (111)planes, however these (111) planes meet and form convex edges and sharptips, which are etched in a much faster rates than regularcrystallographic planes. As a result, the exposed semiconductor (111)planes are etched from their peripherals and finically etched through.The see-through hole opening size can be in-situ controlled by using alight source and an optical detector to measure the light transmissivityduring etching and terminated the etching process when a pre-determinedopening size/ratio. After the KOH etching, the remaining hard maskingmaterial is stripped and 3-D TFSS 350 with see-through hole 358 iscleaned and dried (as shown in FIG. 21D).

FIG. 22A shows the schema vie top view of the 3-D TFSS prior to thesee-through hole etching process, in which the large pyramid cavitieswill be selectively patterned and etched from their backsides to formsee-through holes. FIGS. 22B through 22D illustrate a method of formingthe see-through holes from a released 3-D TFSS by etching largepyramidal cavities.

FIG. 22B is a depiction of a cross-section alone the C-C dash line inFIG. 22A. It is important to note that the height/depth of the invertedpyramidal cavities difference between the large and small pyramids is animportant factor for the see-through opening etch and it is determinedby the lateral size difference (designed in the template layout) betweenthe large and small pyramids. There is a minimum pyramid heightdifference requirement for conducting subsequent etching process. InFIG. 22B, a 50 um pyramid height difference is shown. In FIG. 228, ahard mask layer is deposited to cover the front surface (hard mask layer362), back surface (364) and side surfaces of 3-D TFSS 360 comprisingand epitaxial semiconductor layer, a top emitter doping region, and abottom base doping region. The hard mask material is used to protectsurfaces from the subsequent semiconductor etching, such a KOH etching.The mask materials include but not limited to thermally grown oxide andLPCVD nitride.

Next, as shown in FIG. 22B, the hard mash layer at she large pyramidbottom, shown as 365, is selectively removed by self-aligned methods,such as mechanical lapping or LTC coating of an etch paste. In themechanical lapping case, since the large pyramids have uniform depth,the lapping surface only makes physical contacts to the bottom of thelarge pyramids. Therefore it is convenient to selectively remove thehard mask layer that is less than 1 um thick in this self-alignedprocess. In the second case, a hard mask etching paste or liquid couldbe readily coated to the large pyramid bottom surfaces withoutcontacting the surfaces or the small pyramids. The backside selectiveetchant coating is conducted by a controlled LTC process, which includesof roller coating, dip coating, and screen printing.

After opening the hard mask layer at the bottom surface of the largepyramids, KOH semiconductor etching is used to form the backsideopenings. As shown in FIG. 22C, the etching of the openings progressesalong the sidewalls of the large pyramids and etching is terminatedafter a pre-determined opening sire is reached, such as at point 368. Itis important to note, the amount of the large pyramid etched shouldcreate a reversed height difference of a minimum value between the largeetched opening level and the bottom of the small pyramids. A 50 um of aheight difference is shown in FIG. 22C. This reversed height differencebetween the large and small pyramids is needed for conducting downstreamself-aligned cell fabrication process including passivation layerselective opening and selective base metallization at the bottom of theextruded small pyramids. As shown in FIG. 22C, as the semiconductoretching progresses, overhanging hard mask layer may be left at thesemiconductor etching fronts and affects the etching depth and rateuniformity. Therefore, the semiconductor etching is preferably performedwith agitation of the etching solution, such as megasonic/ultrasonic orjetting mechanisms in order to break off the overhanging hard mask layerat semiconductor etching front. The see-through opening size may bein-situ controlled by using a light source and an optical detector tomeasure the light transmissivity during etching and terminated theetching process when a pre-determined opening size/ratio. After the KOHetching, the remaining hard masking material is stripped and 3-D TFSS360 with see-through hole 370 is cleaned and dried (as shown in FIG.22D).

FIG. 23A shows the schematic top view of the 3-D TFSS prior to thesee-through hole etching process, in which flat surfaces areas among theinverted pyramidal cavities will be selectively patterned etched fromtheir backsides. FIGS. 23B through 23D illustrate a method of formingthe see-through holes from a released 3-D TFSS by etching the flatsurfaces areas of the 3-D TFSS.

FIG. 23B is a depiction of a cross-section along the C-C dash line inFIG. 23A. In FIG. 23B, a hard mask layer is deposited to cover the frontsurface (hard mask layer 382), back surface (384) and side surfaces of3-D TFSS 380 comprising and epitaxial semiconductor layer, a top emitterdoping region, and a bottom base doping region. The hard mask materialis used to protect surfaces from the subsequent semiconductor etching,such a KOH etching. The mask materials include but not limited tothermally grown oxide and LPCVD nitride. As shown in FIG. 23C, soft maskmaterial 386, such as photoresist, is spray-coated on the front surfaceand photolithographic defined to open the flat area. The opening patternin photoresist is transferred to the hard mask by etching the exposedhard mask. For example, a thermal oxide hard masked can be etched bybuffered HF solution. After stripping the soft mask layer, KOHsemiconductor etching is used to form the defined openings on the flatarea. In this case, the exposed semiconductor surface is (100)crystallographic plane and its etching yield a tapered through hole with(111) planes on its sidewalls. After the KOH etching, the remaining hardmasking material is stripped and 3-D TFSS 380 with see-through hole 388is cleaned and dried (as shown in FIG. 23D).

FIGS. 24A through 24D are cross-sectional drawings depicting a processflow for the fabrication a 3-D TFSC with or without see-through holes.The process steps include surface passivation on front and back surfacesof the released 3-D TFSS, anti-reflection layer coating, selective andlocal dielectric layer opening, emitter and Case metal plating, and backreflector metal deposition.

In FIG. 24A, a thin surface passivation layer is deposited on allexposed surfaces of 3-D TFSS 400 comprising see-through hole 408. As anexample, a thin semiconductor oxide (SiO₂) 404 of 3 nm to 100 nm thickmay be thermally grown by oxidation in the temperature range of 800° C.to 950° C. to serve as the passivation layer for both the front and backsides. Next, a 3 nm to 100 nm thick PECVD semiconductor nitride(SiN_(x)) is deposited on the front side oxide surface for serving as ananti-reflection (ARC) layer and another front surface passivationlayer—shown as front ARC and passivation layer 402. Alternative or inaddition to the thin thermal oxide, a thin layer of reactive sputteredor PECVD deposited aluminum oxide (AlO_(x)) or PECVD SiN_(x) can be usedfor the back surface passivation—shown as backside passivation layer404.

Next, as shown in FIG. 24B, the emitter contact areas 410 and basecontact areas 412 are subsequently opened by spatially selective etchingof dielectric layer(s). In one embodiment, dielectric etching liquid,such as diluted HF, is roller coated or dip coated to the front ridge orthe back, pyramid tip regions for direct etching the dielectric layers.After the etching, the 3-D TFSS is rinsed with DI water and dried. Inanother embodiment, the dielectric etching paste is screen printed onthe front ridge or the back pyramid, tip regions followed by a heatedcuring step. During the heating of the coated etch paste, the dielectriclayer is etched,

After the etching, the remaining etch paste is stripped and the 3-D TFSSis cleaned as shown in FIG. 24C. Other LTC methods disclosed U.S. PatentApplication Pub. No. 2009-0042320 (which is incorporated by reference)may also be used for this spatially selective etch liquid/past coating.In yet another embodiment controlled laser ablation with short-pulsed(Pico-second) UV laser is used, to selectively remove the dielectriclayers. In yet another embodiment, photolithography patterning of theemitter and base contact opening could be conducted. In this method,photoresist is sprayed and dried following by aligned exposure andresist developing. Upon creating the contact opening pattern byphotoresist, chemical etching of the exposed dielectric layers could beconducted. Alter that the remaining resist layer is removed. The abovemethods of spatially selective etching of dielectric layers could beused for both emitter and base contact openings however tine etchingsource material may be different. After the emitter and base contactopenings, an optional aluminum (Al) deposition by PVD, evaporation orsputtering is conducted on TFSS backside followed by anannealing/fire-through at 800° C. to 950° C. The annealing processprovide Al—Si alloy and form region at base contact 414. The remainingaluminum at TFSS backside can be optionally removed.

Next, as shown in FIG. 24D, the emitter and base metallization isconducted by selective electroplating or electroless placing of nickel(Ni) in the thickness range of 50 nm to 300 nm followed by optional Nisintering. Next, electroplating or electroless plating of Ag isconducted on both the emitter contact regions 416 and base contactregions 418. The Ni and Ag plating be replaced by Cu plating with asuitable barrier layer (e.g. Ta). Next, an optional forming gasannealing (FGA) is performed at temperature range of 300° C. to 400° C.to reduce the interconnect resistance and help with surface/bulkpassivation of metals. Next, a thin layer of Al is deposited byevaporation or sputtering on the TFSC backside to serves asback-reflection mirror 420. Optionally, the FGA process could beperformed at this stage. FIG. 24D illustrates the fully processed 3-DTFSC to be packaged in a solar module or panel.

FIGS. 25A through 25D are cross-sectional drawings depicting a processflow for the fabrication a 3-D TFSC with or without see-through holes.The process steps include surface passivation on front and back surfacesof the released 3-D TFSS, anti-reflection layer coating, screen printedemitter and base metal contacts, and back reflector metal deposition.

As shown in FIG. 25A, a thin surface passivation layer is deposited onall exposed surfaces of 3-D TFSS 430 comprising see-through hole 436. Asan example, a thin semiconductor oxide (SiO₂) of 3 nm to 100 nm thick isthermally grown by oxidation in the temperature range of 800° C. to 950°C. to serve as the passivation layer for both sides. Next, a 3 nm to 100nm thick PECVD semiconductor nitride (SiN_(x)) is deposited on the frontside oxide surface for serving as an anti-reflection (ARC) layer andanother front surface passivation layer—shown as front ARC andpassivation layer 432. Alternatively or in addition to the thin thermaloxide, a thin layer of reactive sputtered or PECVD deposited aluminumoxide (AlO_(x)) or PECVD SiN_(x) can be used for the back surfacepassivation—shown as backside passivation layer 434.

As shown in FIG. 25B, backside (base) metallization 438 is performed bycoating metal (aluminum) paste or like to the backside pyramid tipregions. The Al paste can be roller coated, dip coated or screen printedin a self-aligned coating process, i.e. only the pyramid tip regions arecoated. After drying/curing of the coated metal paste by IR or ovenheating, the base metal fire-through process may be optionally performedso that the Al paste can be fired through the backside dielectriclayer(s) and form direct metal to semiconductor base contact;alternatively, this firing step may be postponed and combined with theemitter firing process as a single co-firing step.

Next, as shown in FIG. 25C, emitter metallization coating is performedin two optional paths. In the first option, metal (Ag) paste isroller-coated, dip-coated, or screen-printed to the top ridge portionsof the front striate in a self-aligned manner. Again, otherliquid-transfer-coating methods disclosed in U.S. Patent ApplicationPub. No. 2003-0042320 (which is incorporated by reference) may also beused for the case and emitter metal paste coating process. In the secondoption, the metal paste is screen-printed with alignment to certainfeature on the 3-D TFSS, as results, only specific area, such as stripesof flat areas are coated with the metal paste. Next, the coated emitter(Ag) metal paste is dried and fired through the front side surfacepassivation and ARC dielectric layers to form emitter contact 440. Afterthe front and back metal fire-through process, an optionalelectroplating or electroless plating of an Ag layer is conducted forincreased emitter and base conductivity. The Ni and Ag plating may bereplaced by Cu plating with a suitable barrier layer (e.g. Ta).

Next, as shown in FIG. 25D, a thin layer of Al is deposited byevaporation or sputtering on the TFSC backside to serves asback-reflection mirror 442. Next, an optional forming gas annealing(FGA) is performed at temperature range of 200° C. to 400° C. to reducethe interconnect resistance and help with surface/bulk passivation ofmetals. FIG. 25D illustrates the fully processed 3-D TFSC to be packagedin a solar module or panel.

FIGS. 26A and 26B are a cross-sectional depiction illustrating liquidtransfer coating with a roller coater and the resulting 3-D TFSS. TheLTC process shown uses a selective and self-aligned liquid transfercoating (LTC) process in which a liquid/paste thin layer is coated onthe pyramid tips on backside at 3-D TFSS 450. As shown in FIG. 26A,rollers 454 are placed in a parallel roll and partially merged in stores452 of the liquid/paste to be coated. The size/diameter and pitch or therollers are not drawn to scale FIG. 26A at it is much larger than thepyramid dimensions. The rollers also serve as a conveyer, on which 3-DTFSS 350 is placed and moved when rollers 454 turn. Alternatively, the3-D TFSS could be mounted/vacuum-chucked to a carrier plate on its topand actively positioned to move the 3-D TFSS relative to the rollersurfaces. As a roller turns, a thin liquid layer is coated onto theroller surface by surface forces; the liquid layer thickness depends ona variety of parameters including liquid viscosity, surface tension,temperature, roil surface condition, roller rotation speed, 3-D TFSSproximity distance and its lateral translation speed. These parameterscould be actively controlled so that upon contact between the liquid onthe roller top surface and the 3-D TFSS, a controlled-amount of liquidis transferred to the large pyramid tips 456 on the 3-D TFSS backside.

As shown in FIG. 26B the liquid is coated 458 on the large pyramid tipsof 3-D TFSS 450 in a self-aligned fashion. As described previously, thepresent LTC method may be applied to (1) emitter and base doping, inwhich case there are coating liquid sources an n-type doping and p-typedoping respectively; (2) passivation layer etching, in which case thecoating liquid may be diluted HF solution or oxide/nitride etch pastes;(3) semiconductor etching, in which case the coating liquid may be KOHsolution or HNA like solutions; (4) local emitter and basemetallization, in which case the coating liquid is a metal paste or aliquid with dissolved metal nano-particles.

FIGS. 27A and 27B are a cross-sectional depiction illustrating liquidtransfer coating according to a stamping/dipping process. Shown, aliquid/paste thin layer is coated on the selective surfaces of 3-D TFSS460 from a liquid carrier or surface such as a screen or a pad. As shownin FIG. 27A, the front pyramid ridges and back pyramid tips of a 3-DTFSS are contacting two flat surfaces—front side liquid carrier 462 andback side liquid carrier 464. Thus, the liquid types of the front andback contacts may be the same or different. The choices of liquidcarriers include a pre-soaked porous plate/surface, a fine-grid screen,or a sponge-like solid pad, and a flat surface with pre-coated thinlayer of the liquid. The transferring of liquid from the carrier surfaceto the pyramid surface upon contacting is driven by surface forces ofliquid and/or surface forces between the liquid and pyramid interlaces.The amount of liquid transferred is determined and limited by the saidsurface forces and the amount of liquid pre-soaked on the carrier priorto contacting the 3-D TFSS.

As shown in FIG. 27B, after the liquid transfer coating, the coatedliquid is dried and cured and it only lies on the selective areas frontside areas 466 and selective back side areas 468 of 3-D TFSS 460. Asdescribed in the earlier, the present LTC method can be applied to (1)emitter and base doping, which results in liquid coating of an n-typedoping source and a p-type doping source respectively; (2) passivationlayer etching, in which case the coating liquid may be diluted HFsolution or oxide/nitride etch pastes; (3) semiconductor etching, inwhich case the coating liquid may be KOH solution or HNA like solutions;(4) local emitter and base metallization, in which case the coatingliquid is metal pastes or a liquid with dissolved metal nano-particles.

FIGS. 28A and 28B are a cross-sectional depiction illustrating aselective and self-aligned mechanical polishing/lapping process in whicha thin layer of a material on pyramid surfaces of a 3-D TFSS is removed.As shown in FIG. 28A, 3-D TFSS 462 is sandwiched between two parallelplates/surfaces. Bottom plate 488 is a vacuum chuck and top mountingplate 480 has abrasive surface 482 (such as a lap pad) for lapping thetop surface of 3-D TFSS 486, 3-D TFSS 486 is optionally submerges; in aliquid, such as DI water 484, for the convenient carrying and removingof lapping debris. Alternatively, the liquid may be a diluted chemicaletchant or slurry for assisting the mechanical removal of the surfacelayer. Abrasive surface 482 may be a soft lapping pad or a fine gridlapping cloth, which is used to gently and evenly conduct the mechanicalpolishing/lapping in a controlled lateral and/or rotation motions.

Since the pitch between pyramid tips being polished is less than 1 mm,the local deformation of the lapping pad between the two adjacentpyramid tips is smaller than the height difference between the large andsmall pyramids. FIG. 28A is not drawn to scale as the thickness of theplates is much larger than the pyramid dimensions. In this mechanicallapping process, the choice of abrasive materials, the roughness andelasticity of she lapping surface, the force loaded in the verticaldirection (weight) onto the 3-D TFSS, lateral and rotation speeds needto be managed so that the mechanical damage to the 3-D TFSS isprevented.

Resulting 3-D TFSS 486 is shown in FIG. 28B—a thin surface layer is onlyremoved from the large pyramid tip surfaces 490 with the small pyramidtips untouched. Mechanical lapping method like this may be applied toeither front or backside surfaces. Therefore, as described in theearlier, this method of selective and self-aligned thin layer removalcan be used in (1) opening the hard-etch-mask on backside at largepyramid tips tor see-though hole etching; (2) Opening hard mask layer onfront and/or backside surfaces for selective emitter/base doping; and(3) Opening the passivation dielectric layer on front and/or backsidesurfaces for selective emitter/base metallization.

FIGS. 29A and 29B are a crocs-sectional depiction illustrating aselective and self-aligned mechanical polishing/lapping process in whichthin layer of a material on pyramid surfaces of a 3-D TFSS is removed bya diaphragm based abrasive lapping. As shown in FIG. 29A, 3-D TFSS 502is held by vacuum chuck 504 and optionally submerged in liquid 500, suchas DI water, for convenient carrying and removing lapping debris.Alternatively, the liquid may be a diluted chemical enchant or slurryfor assisting in the mechanical, removal, of the surface layer. Abrasivematerial 508 makes contact to the surfaces of 3-D TFSS 502 and may be asoft lapping pad or a fine grid lapping cloth which is used to gentlyand evenly conduct the mechanical polishing/lapping in a controlledlateral and/or rotation motions. The local surface of abrasive material508 that makes contact to the TFSS surface is slightly deformed in orderto achieve uniform thin film removal. Diaphragm 512 and compressedair/elastic material 510 provide structural yet flexible support toabrasive material 508. Since the pitch between pyramid tips on 3-D TFSS502 being polished are less than 1 mm, the local deformation of thelapping pad between the two adjacent pyramid tips is smaller than theheight difference between the large and small pyramids. FIG. 29B is notdrawn to scale as the curvature of the abrasive diaphragm is much largerthan the pyramid dimensions. In this mechanical lapping process, thechoice of abrasive materials, the roughness of the lapping surface, theelasticity of the diaphragm, the slurry type, the liquid temperature,the force/pressure loaded in the vertical direction (weight) onto the3-D TFSS, lateral and notation speeds need to be managed so that thethin layer on the TFSS surface can be effectively removed without damageto the 3-D TFSS structure.

As shown in resorting 3-D TFSS 502 in FIG. 29B, a thin surface layer isonly removed from the large pyramid tip surfaces 512 and the small,pyramid tips are untouched. Mechanical lapping method like this can beapplied to either front or backside surfaces. Therefore, as described inthe early sections, this method of selective, and self-aligned thinlayer removal can be used in (1) Opening the hard-etch-mask on backsideat large pyramid tips for see-though hole etching; (2) Opening hard masklayer on front and/or backside surfaces for selective emitter/basedoping; (3) Opening the passivation, dielectric layer on front and/orbackside surfaces for selective emitter/base metallization.

FIGS. 30A and 30B are a cross-sectional depiction illustrating aselective and self-aligned mechanical polishing/lapping process in whicha thin layer of a material on pyramid surfaces of a 3-D TFSS is removedby mechanical abrasive tape based lapping. As shown in FIG. 30A, 3-DTFSS 522 is held by vacuum chuck 524 and optionally submerged in liquid520, such as DI water, for convenient carrying and removing lappingdebris. Alternatively, the liquid could be a diluted chemical etchant orslurry for assisting the mechanical removal of the surface layer.Abrasive tape 526 is pressed on the TFSS surface to be polished by softroller 528 and the tape is fed to the surface by a tape-feedingmechanism (not shown). Together with roller 528 roller and thetape-feeding mechanism, the tape feeding speed, the roller rotationspeed, and the lateral translation of the roller-tape system is activelycontrolled. Abrasive tape 526 is soft (such as a PET film) and it hasabrasives (such as particles of semiconductor carbide, aluminum oxide,chromium oxide, diamond, and semiconductor oxide) embedded/adhered toits surface so that it can be used to gently and evenly conduct themechanical lapping. Since the pitch between pyramid tips being polishedis less than 1 mm, the local deformation of the lapping pad between thetwo adjacent pyramid, tips is smaller than, the height differencebetween the large and small pyramids. FIG. 30A is not drawn to stale asthe diameter of the roller is much larger than the pyramid dimensions.In this mechanical lapping process, the choice of abrasive materials,the roughness of the lapping tape surface, the slurry type, the liquidtemperature, the force/pressure loaded in the vertical direction(weight) onto the 3-D TFSS, tape-feeding speed need to be managed sothat the thin layer on the TFSS surface can be effectively removedwithout damage to the 3-D TFSS structure.

As a result, and as shown in FIG. 30B, a thin surface layer is onlyremoved from the large pyramid tip surfaces 530 of 3-D TFSS 522 with thesmaller pyramid, tips untouched. This mechanical lapping method may beapplied to either front or backside surfaces. Therefore, as described,in the earlier, this method of selective and self-aligned thin layerremoval may be used in (1) Opening the hard-etch-mask on backside atlarge pyramid tips for see-through hole etching; (2) Opening hard masklayer on front and/or backside surfaces for selective emitter/basedoping; (3) Opening the passivation dielectric layer on front and/orbackside surfaces for selective emitter/base metallization.

FIGS. 31A and 31C are a cross-sectional depiction illustrating atemplate/wafer edge lapping process in which a thin layer of a materialat the template edge is removed by an abrasive tape based lapping.

As shown in FIG. 31A, abrasive tape 546 in pressed on the wafer edge bytwo rollers that are place in a distance away from the wafer edge. Thewafer comprising template 544, porous semiconductor layer 542, andepitaxial layer 540. During lapping, abrasive tape 546 is continuouslyfed and guided, by the two rollers 548. The distance and the angle ofthe two rollers 548 with respect to the wafer edge location determinesthe abrasive tape contact location, angle, and area, thus determines theprofile of the wafer edge after lapping. It is to be noted that thedistance and angle of the two rollers may be actively changed andcontrolled so that various amount of semiconductor at various edgelocations may be removed accordingly. As a result, either a symmetricalor an asymmetrical wafer edge profile may be achieved by this wafer edgelapping method.

The following aspects of this edge lapping method than are integral,parts of the current invention but are not illustrated, in FIG. 31A. (1)Wafer mounting and rotation mechanism: the wafer can be mounted on avacuum chuck with it edge area exposed or it can be clamped between twoplates with the wafer edge exposed. The wafer is rotating in a uniformand controlled speed during lapping; (2) Abrasive tape feeding andtension control mechanism: the abrasive tape is continuously fed so thatthe tape can be actively cleaned/reconditioned before making contact tothe wafer surface. The feeding speed and. tension of the abrasive tapeis controlled by mechanical and electrical systems that consists ofmultiple rollers of different sizes, motors, sensors and controllers;(3) Roller position, and angle control mechanism: the relative positionand angle of the on rollers shown in FIG. 31A are also controlled bymechanical and electrical means including actuators, motors, sensors andcontrollers; and (4) Liquid/chemical/slurry dispensing mechanism: liquidsuch as DI water, chemicals such as diluted semiconductor enchant,and/or abrasive slurry can be added to the wafer edge area where contactis made to the abrasive tape. These lapping medium can be added bydirect dispense for a nozzle in proximity to the lapping area orcarried, to the lapping area by the abrasive tape.

The lapping tape is soft (such as a PET film) and it has abrasives (suchas particles of semiconductor carbide, aluminum oxide, chromium oxide,diamond, and semiconductor oxide) embedded/adhered to its surface sothat it can be used to gently and evenly conduct the mechanical lapping.FIG. 31A is not drawn to scale as the diameter of the roller could belarger than the wafer thickness. In this mechanical lapping process, thechoice of abrasive materials, the roughness of the lapping tape surface,the slurry type, the liquid temperature, the force/pressure loaded tothe wafer edge, tape tension and tape-feeding speed, need to be managedso a thin layer of the wafer edge can be effectively removed, withoutdamage to the wafer integrity.

FIG. 31B illustrates the edge tape lapping method for facilitating the3-D TFSS release and template re-use. Pre-structured semiconductortemplate 544 has grown epitaxial semiconductor layer 540 on top of abi-layer porous semiconductor thin layer 542 in accordance with thepresent application. The edge lapping method is used for (1) to removethe edge wrap-around epitaxial semiconductor layer so that it would notbe locked to the template; (2) to expose the buried porous semiconductorlayer at edge—shown at wafer edge 550. Many 3-D TFSS release mechanismsdescribed herein require the initiation of the releasing/separation ofthe porous semiconductor layer at the wafer edges. With this polishededge, the exposed porous semiconductor layer at the wafer edges servesas the releasing/separation initiation locations; (3) to polish thetemplate edge in order to remove the semiconductor nodules/roughnessfrom the porous semiconductor formation and epitaxial growth processes.A polished smooth template edge will make the template more robust forsubsequent handling and re-use cycles as described. FIG. 31B illustratesthe polished edge 560 of the wafer.

Next, the 3-D TFSS may be released/separated from the template. FIG. 31Cillustrates released 3-D TFSS 552 and template 544. The poroussemiconductor residue on the released TFSS and template surfaces areremoved by diluted semiconductor etching solution, such as TMAH or KOHor NaOH. It is to be noted, the wafer edge tape lapping method may beused to polish a wafer of circular shape as well as wafers with squareshape or quasi-square shapes. In the non-circular edge tape lappingcases, the wafer may not be rotating against its central axis; instead,the motion of the lapping tape is controlled to follow the wafer edgeprofile in the lateral directions.

FIG. 32 is a process flow outlining the major steps or an overallre-usable template, 3-D TFSS and TFSC fabrication process with selectivebase metal, epitaxial emitter and plated metallization. The re-usabletemplate making process as shown in the top section of FIG. 32 is sameas described in FIG. 9 and FIGS. 15A through 15D. The Epitaxial EmitterPycell (3-D TFSS) Substrate process is similar to the process describedin FIG. 10 and FIGS. 16A through 16C. The term “Pycell” is refers topyramid 3-D TFSS. In the backend Pycell (3-D TFSS) process as shown inFIG. 32, an optional thin thermal oxide layer of less than 100 A thickis first grown on the 3-D TFSS surfaces for passivation. Thenhydrogen-rich PECVD semiconductor nitride is deposited on front and backside surfaces. In addition to serving as passivation layers, thesemiconductor nitride on front side may also used as antireflectioncoating (ARC) and the semiconductor nitride on the backside may also beused as back surface reflection (BSR) layer.

In the next step, dielectric etch paste/liquid is coated on the frontand back surface in a self-aligned coating process, such as rollercoating or screen printing. The etch paste/liquid is selectively coatedon the front ridges and back pyramid tips, where the dielectric layer isto be opened locally. After coating, the paste is cured/baked toactivate the dielectric layer etching. After the etching, the residuepaste and removed and the 3-D TFSS is cleaned. Next, a thin Al or NiV(nickel vanadium) layer is sputtered on the 3-D TFSS backside to serveas a backside surface reflection (BSF) layer. Next, electroless Niplating is conducted on the front and back sides followed by LIP (lightinduced plating) of Ag on the front side. After the plating, a lowtemperature annealing is conducted for reducing the contact resistance.

FIG. 33 is a process flow outlining the major steps of an overallre-usable template, see-through 3-D TFSS and TFSC fabrication prose orwith selective base metal, epitaxial emitter and plated metallization.The re-usable template making process as shown in the top section ofFIG. 33 is same as described in FIG. 9 and FIGS. 15A through 15D. TheEpitaxial Emitter Pycell (3-D TFSS) Substrate process is similar to theprocess described in FIG. 10 and FIGS. 16A through 16C. The term“Pycell” is refers to pyramid 3-D TFSS. After the 3-D TFSS releasing andits backside QMS (quasi-mono-crystalline semiconductor) layer removal,the see-through hole openings are made by methods described in FIG. 10,FIGS. 21A through 21D, FIGS. 22A through 22D, FIGS. 23A through 23D, andFIGS. 24A through 24D. In the backend Pycell (3-D TFSS) process as shoesin FIG. 33, an optional thin thermal oxide layer of less than 100 Athick is first grown on the 3-D TFSS surfaces for passivation. Thenhydrogen-rich PECVD semiconductor nitride is deposited on front and backside surfaces. In addition to serving as passivation layers, thesemiconductor nitride on front side may also used as antireflectioncoating (ARC) and the semiconductor nitride on the backside may also housed as back surface reflection (BSR) layer. In the next step,dielectric etch paste/liquid is coated on the front and back surface ina self-aligned coating process, such as roller coating or screenprinting. The etch paste/liquid is selectively coated on the frontridges and back pyramid tips, where the dielectric layer is to be openedlocally.

After coating, the paste is cured/baked to activate the dielectric layeretching. After the etching, the residue paste and removed and the 3-DTFSS is cleaned. Next, a thin Al or NiV (nickel vanadium) layer issputtered on the 3-D TFSS backside to serve as a backside surfacereflection (BSF) layer. Next, electroless Ni plating is conducted on thefront and back, sides followed by LIP (light induced plating) of Ag onthe front side. After the plating, a low temperature annealing isconducted for reducing the contact resistance.

The foregoing description of the exemplary embodiments is provided toenable any person skilled in the art to make or use the subject matter.Various modifications to these embodiments will be readily apparent tothose skilled in the art, and the generic principles defined herein maybe applied to other embodiments without the use of the innovativefaculty. Thus, the subject matter to be claimed in subsequently filedapplications is not intended to be limited, to the embodiments shownherein but is to be accorded the widest scope consistent with theprinciples and novel features disclosed herein.

What is claimed is:
 1. A three-dimensional thin-film solar cell,comprising: a plurality of at least two differently sizedthree-dimensional inverted pyramidal cavities comprising a larger set ofthree-dimensional inverted pyramidal cavities and a smaller set ofthree-dimensional inverted pyramidal cavities wherein said larger set ofinverted pyramidal cavities and said smaller set of inverted pyramidalcavities are arranged in a staggered pattern on a semiconductorsubstrate; a plurality of selectively formed through-holes in saidsemiconductor substrate with openings between the front and back lateralsurface planes of said semiconductor substrate which form a partiallytransparent three-dimensional thin-film semiconductor substrate, whereinsaid semiconductor substrate has an inverted pyramidal structure with alayer thickness in the range of 1 to 60 microns and a substrate peak topeak thickness in the range of approximately 100 to 500 microns; a sunnyside doped emitter region positioned on the top surface of saidsemiconductor substrate; a back surface field p-type doped base regionpositioned on the bottom surface of said semiconductor substrate; aplurality of emitter metal contacts positioned on base openings of saidinverted pyramidal cavities on said semiconductor substrate; and aplurality of base metal contacts positioned on the bottom surface ofsaid semiconductor substrate at the apex of said inverted pyramidalcavities on said semiconductor substrate.
 2. The three-dimensionalthin-film solar cell of claim 1, wherein said plurality of selectivelyformed through-holes in said semiconductor substrate are positioned insaid smaller set of three-dimensional inverted pyramidal cavities onsaid semiconductor substrate.
 3. The three-dimensional thin-film solarcell of claim 1, wherein said plurality of selectively formedthrough-holes in said semiconductor substrate are positioned in saidlarger set of three-dimensional inverted pyramidal cavities on saidsemiconductor substrate.
 4. The three-dimensional thin-film solar cellof claim 1, wherein said plurality of selectively formed through-holesin said semiconductor substrate are positioned on the top surface areason said semiconductor substrate.
 5. The three-dimensional thin-filmsolar cell of claim 1, further comprising a front side surfacepassivation layer positioned on said doped emitter region.
 6. Thethree-dimensional thin-film solar cell of claim 5, wherein said frontside surface passivation layer is a thermal oxide.
 7. Thethree-dimensional thin-film solar cell of claim 5, further comprising ananti-reflection layer positioned on said doped emitter region.
 8. Thethree-dimensional thin-film solar cell of claim 1, further comprising aback side surface passivation layer positioned on said base region backsurface field p-type doped base region.
 9. The three-dimensionalthin-film solar cell of claim 8, wherein said back side surfacepassivation layer is a thermal oxide.
 10. The three-dimensionalthin-film solar cell of claim 1, wherein said sunny side doped emitterregion is an n-type doped emitter region.